2017-2018 Winter Synopsis

This brief outlook for the winter will be extracted from global sea surface temperatures.

ESRL

1) La Nina
As has been expected for a while, a La Nina has now emerged. In the image above, we see a stripe of below-normal sea surface temperature (SST) anomalies from the coast of Ecuador to the 180-degree longitude line. The deepest negative anomalies appear to be centered near the 120-degree west line of longitude, also known as the eastern part of the Nino 3.4 region, and extending a bit west into the center of the Nino 3.4 region. As such, this looks to be a 'textbook' La Nina, though then again rarely is anything in meteorology "textbook".

CPC

If we look at a cross-section of temperature anomalies along the Equator down to 450 meters below the surface, we see how this pool of below-normal waters has slowly but surely drifted up to the surface from ~150 meters under the surface. After struggling to do so in October, the cold pool has solidified itself on the surface, as we already saw in the top image.

I show this animation to provide evidence that this La Nina looks to be a formidable one, in the sense that it has a reservoir of below-normal temperature waters to continue gradually propagating to the surface. Previous La Nina and El Nino events have been foiled by the lack of a sturdy foundation like this, while others have been eroded away by a body of opposing-temperature waters out to the west, like is seen currently from the surface to 200 meters below. It remains to be seen if this body of colder waters will follow in those footsteps and be eroded over time, but for now it looks rather well-positioned to stick around into the spring.

Effects

ESRL
Typical precipitation anomalies in a La Nina

ESRL
Typical temperature anomalies in a La Nina

The above two images show the typical effects seen from a La Nina in precipitation, on top, and temperature, on bottom. In a La Nina regime, above-normal precipitation is generally seen in the Pacific Northwest, with drier than normal weather observed from southern California through the southern Plains, Gulf Coast and up into the Mid-Atlantic. Some dry signals of a lighter magnitude are also evident in the Midwest, but these are opposed by a swath of wetter than normal conditions generally seen in La Nina years, in the Ohio Valley.
In terms of temperature, cooler than normal conditions are typically observed from the Pacific Northwest through the Upper Midwest, just barely through Maine to the Atlantic coast. Warmer than normal conditions typically dominate the Southern U.S., most strongly in the Southeast U.S. and Texas and Louisiana.

But how do these effects come to pass?

ESRL
Typical 500mb geopotential height anomalies in a La Nina

In past La Nina winters, the mid-level flow has shaped up with a strong negative anomaly in western Canada, in what appears to be a core of cold and snowy weather. This is opposed by a strong ridge of high pressure, centered south of the Aleutian Islands but slanted so that it stretches from extreme northeast Russia to the waters just offshore of Baja California.

It is this ridge that shapes the temperature and precipitation anomalies. The Pacific jet stream rides the line between these two extremes, generally pushing southward from Alaska or the Gulf of Alaska, typically then beginning a northward turn when it hits central California. Note the slight positive anomaly in the above image along the Gulf Coast. This is a signal (albeit a very weak one) of the infamous Southeast Ridge that can emerge in La Nina years.

The Southeast Ridge is what I like to call the ridge of high pressure that will typically form over the Southeast in La Nina winters, leading to that maxima of warmer than normal temperatures in that region, but also pushing the jet stream north slightly as that ridge forms to divert the storm track through the Ohio Valley, leading to - you guessed it - the swath of above-normal precipitation usually seen in the Ohio Valley in La Nina winters.

Prediction: La Nina continues through winter and into the spring of 2018.

2) Bering Sea / North Pacific
Another point of interest on that sea surface temperature anomaly at the top of this publication is the patch of above-normal SSTs south of the Gulf of Alaska, in the northeast Pacific. Note how we see that swath of warmer waters seemingly corralled by average or slightly below-average water temperatures that appear to be hugging the Pacific Northwest/Alaska coast. If you think this looks like the negative phase of the Pacific Decadal Oscillation (PDO), you'd be right.

NCSU
Typical sea surface temperature anomalies in a negative PDO

As the above image shows, the negative phase of the PDO is identified primarily by a body of warmer than normal waters south of the Gulf of Alaska, surrounded by colder than normal waters along the coast of North America.

How does this tie in to the La Nina? I'm glad you asked!

University of Washington
Broader view of SST anomalies in the warm phase (positive) and cold phase (negative) PDO

As the above image shows, the warm phase of the PDO is generally associated with the El Nino phenomenon, as observed by the warmer colors along the Equator. Similarly, in a negative PDO, La Nina tends to dominate, with the composite image above showing cooler colors (and hence cooler water temperatures) along the Equatorial Pacific. As the sea surface temperatures suggest, we have seen the PDO as negative since July of this year.

NCEI
Graph showing recent values of the PDO

Effects

http://la.climatologie.free.fr/enso/enso-pdo3-english.htm#pdo

Since the negative phase of the PDO is generally seen with La Nina conditions in the Equatorial Pacific, you might expect the effects of a negative PDO to be similar to that of a La Nina. And you'd be right! As the above image shows, the negative phase of the PDO is shown to generally produce wetter than normal conditions in the Pacific Northwest, drier than normal conditions in the South and Southeast, and wetter than normal conditions in the Ohio Valley. This is remarkably similar to precipitation effects observed in a La Nina, and as such, the forecast adjusts to make this winter look more like a "textbook" La Nina winter.

Prediction: the negative PDO regime stays in place through the winter, though the lack of strong negative SST anomalies along the North American coast makes me wonder if this regime will stay in place through the spring of 2018.

3) Warmth by Nova Scotia
So we should just stop here and say this winter will look more or less like a 'typical' La Nina winter, we should just stop the article here, right? Not so fast. There's something else that's catching my eye.

NHC
Weekly SST Anomalies, valid 11/18/17

There's a swath of much warmer than normal SST anomalies centered just south of Nova Scotia, and this has the potential to become an issue for those in the East hoping for a cooler than normal winter.

In general, SST anomalies can be strong predictors of the presence of ridges or troughs over the longer-term. For instance, warmer than normal waters will tend to promote ridging in the longer-term, while cooler than normal waters will encourage troughing in the longer-term. Consequentially, I'm seeing the possibility for ridging becoming a rather common feature along the East Coast into eastern Canada, due to that swath of warmer than normal waters near Nova Scotia.

It's also entirely possible this actually promotes colder weather for the East, by encouraging ridging that then pushes north into Greenland to promote the negative phase of the North Atlantic Oscillation (NAO). However, the negative NAO is typically encouraged by warmer than normal waters *around* Greenland, not well to the south and west, like by Nova Scotia. Time will tell, of course, but I find this factor to be more likely to encourage a warmer winter for the East than a colder winter.

Prediction: Ridging becomes more prevalent than stormier weather in the Canadian Maritimes, potentially working in tandem with the La Nina / negative PDO-induced Southeast Ridge to push the jet stream and primary storm track a little further north and west, affecting both the Ohio Valley and the Midwest.

4) Warm Waters in the Bering Sea
One final sea surface temperature-related factor to discuss is the body of warmer than normal waters present in the Bering Sea southward into the north-central Pacific.

As we discussed previously, SST anomalies can promote longer-term ridging or troughing patterns. This still applies here in the Pacific, and may also complicate the forecast further. The presence of warmer than normal waters in the north-central Pacific and Bering Sea could promote a strong ridge, perhaps evolving into a blocking pattern, which would then trigger a trough forming in the Gulf of Alaska down along the West Coast, similar to the pattern we are in as I type this:

CPC
Analysis of 500-millibar geopotential height anomalies in five-day tranches over the last month

Over the last month or so, we've seen a persistent blocking ridge in the north-central Pacific into the Bering Sea. However, the troughing downstream of this ridge has been variable, moving from the West Coast from 11/3-11/7 to the waters just south of the Gulf of Alaska from 11/18-11/22. The former period saw stormier weather in the West, while the latter period saw a ridge form.
This goes to show how, even though the ridge remained relatively constant in the Bering Sea region, the effects downstream are not as certain. This will add some more uncertainty to the forecast for the coming winter, should these warmer than normal waters remain in the north-central Pacific and Bering Sea.

Prediction: A ridge present in the Bering Sea would introduce uncertainty to the winter forecast, and likely result in stormier weather along the West U.S., which in turn would most likely result in a warmer forecast for the U.S. as a whole.

To Summarize:

- Currently expecting a cooler than normal winter for the far northern Plains and Upper Midwest into New England as a result of the negative PDO and La Nina.
- Wetter than normal conditions likely for the Ohio Valley and eastern Midwest, with drier than normal conditions expected to prevail in the Southeast, Gulf Coast and Deep South.
- The La Nina & negative PDO combination should dictate the broader weather pattern through the winter, with warmer waters in the Bering Sea encouraging meridional flow in the Pacific, and likely an active Pacific jet.

Graphical Forecast

Andrew

Long-Range Forecast: November 24th - December 21st, 2017

This is a long-range forecast for the November 24th thru December 21st, 2017.

Weeks 1 and 2

We begin with the weekly forecasts from the CFS model, looking at the Week 1 and 2 timeframe.

NCEP / CPC

The top panel shows 500-millibar geopotential height anomalies from November 24th thru November 30th, with the same parameter shown on the bottom panel from December 1st thru December 7th.

We first analyze the top panel, and see a quite warm pattern set up for the country through the end of the month. This warmth stems from the jet stream being forced north as a strong blocking pattern, present since late October (as seen below), has set up in the Pacific just south of the Aleutian Islands.

NCEP / CPC

This blocking pattern has allowed a persistent roughing pattern to emerge in the Gulf of Alaska into the Pacific Northwest, which as you can see from November 9th onward in the above animation, has pumped a ridge up across the South U.S. into the central and eastern states. The East has been increasingly unaffected by this, as the last handful of frames in the animation show a negative North Atlantic Oscillation (NAO) pattern emerge as a strong ridge has blossomed over Greenland. This has allowed a trough to build down through southern Canada into the Northeast and Great Lakes region, keeping that area relatively cooler.

The Week 1 forecast, then, is more or less a continuation of this pattern, but the CFS anticipates the ridge in the South to bleed east and overrule that trough that's trying to push into the Northeast thanks to the negative NAO.

In Week 2, the bottom panel of the top image, we see a dramatically different pattern. The blocking pattern we are currently enduring in the northern Pacific has broken down in Week 2, with the CFS forecasting a strong low in the Bering Sea and a ridge well south of Alaska to promote a stronger Pacific jet stream and more zonal flow. With this ridge seen blossoming east towards the West Coast, that trough currently in the West U.S. is expected to weaken, leaving the Pacific jet with a red carpet rolled out to sweep into the U.S.

As is typically seen in this sort of Pacific pattern, a broad ridge is then forecasted to set up in the Central & Eastern U.S., focused in the Southeast (like one may typically see in a La Nina pattern). What we also see, however, is a very strong negative NAO pattern now having evolved, with a strong ridge over Greenland.
The actual temperature forecast from the CFS for these two weeks reflects the deductions we've made above:

NCEP / CPC

Weeks 3 and 4

NCEP / CPC

Weeks 3 and 4 show much less meridional flow over the Northern Hemisphere, albeit with a reduced magnitude in all troughs and ridges as the members of this ensemble diverge, diluting the mean forecasts shown here.
Most interestingly, we see a relatively strong ridge positioned right in the Arctic Circle, which would suggest a displacement of the (now infamous) polar vortex to the south. Where it goes if it was to be displaced south is very much to-be-determined, but early approximations in that top panel show below-normal anomalies (stormier conditions) in the Northeast U.S., northern Europe and Japan up through the Bering Sea.

Week 4 brings a similar picture to Week 3, though with a weaker ridge over the Arctic Circle and a resurgent ridge in the northeast Pacific and West Coast in tandem with a trough in the Gulf of Alaska. This again seems like a pattern where the Pacific jet stream may dominate, especially with that slight ridge shown well to the east of Florida that may resemble a Southeast Ridge if it keeps showing up in future forecasts. By this time, however, confidence is substantially lower, although the corresponding temperature graphics again are rather similar to what we've discussed above:

NCEP / CPC

Interestingly enough, the CFS isn't biting too hard on the possible cold wave in Week 3 as that ridge establishes itself in the Arctic Circle. We do see some light cooler-than-normal anomalies in the Southeast in this period, but the country as a whole is broadly above-normal.

To Summarize:
- Warmer than normal conditions are expected for the majority of the country through the end of November and the first week of December.
- Colder than normal conditions may present themselves in the Eastern U.S. by the second week of December with the emergence of a ridge in the Arctic Circle, but a strong Pacific jet stream may not permit this cooler weather to stay for a prolonged period of time.

Andrew

La Nina Expected for Winter 2016-2017

A La Nina is expected to develop this summer, and should reach maximum intensity during the winter of 2016-2017.

NCEP/NOAA

A look at sea surface temperature anomalies across the central Pacific reveals a change in progress. Over the last several months, we have been in a notable El Nino event, which is characterized by above to well-above average temperature anomalies in the waters along the Equator, west of Ecuador. However, in the last few weeks, we've started seeing those warmer water anomalies wane, and even be replaced by cooler than normal anomalies. This switch is occurring mainly in the waters immediately offshore Ecuador, as well as a little farther out to sea, between the 120W and 110W longitude lines.

CPC

We can trace this cooling to an upwelling event ongoing across the Equatorial Pacific. Shown is a cross-section diagram of the ocean at the Equator, from a depth of the surface to 450 meters. Ever since February, note how we've been seeing well-below normal water anomalies pushing towards the surface, and recently breaching the surface near Ecuador. This is the root cause behind those cooling anomalies on the surface image discussed earlier in this post, and so long as this upwelling of cooler waters continue, the rest of the Equatorial Pacific should continue to cool down from this El Nino.

But what about the future... specifically the winter ahead?

IRI/CPC

A chart showing the forecasted sea surface temperature anomalies in a portion of the Equatorial Pacific known as Nino region 3,4 shows our decline from an El Nino into 'ENSO-Neutral' conditions around May and June. You may recall that Neutral conditions are when the waters are too warm for a La Nina, but too cool for an El Nino, and this is defined when water temperatures in Nino region 3.4 are between +0.5 degrees anomaly and -0.5 degrees anomaly. We begin to enter a La Nina around August or September, when the average of model guidance dips below -0.5 degrees anomaly, the threshold for a La Nina. You'll also notice it reaches maximum intensity as we head into the winter of 2016-2017.

Now the big question: what does this mean for winter 2016-2017?
The big answer: we don't exactly know. But we do have an estimate of what things will look like if this forecast holds.

NCSU

The above graphic shows typical weather conditions in a La Nina pattern. In the U.S., La Nina's tend to bring cooler than normal conditions to the Pacific Northwest, northern Plains, and Midwest. Wetter/snowier than normal conditions also tend to evolve in the Midwest, Ohio Valley, and portions of the Great Lakes. The Southern U.S., however, generally can expect warm and drier than normal winters out of La Nina's. This isn't a set-in-stone interpretation of what the coming winter will be like, but we could see something similar to this graphic set up.

To Summarize:

- A La Nina is expected to develop this summer, and intensity into the coming winter.
- As is typical in La Nina winters, the Ohio Valley may be wetter/snowier than normal, while cooler weather overspreads the North U.S. The South may remain warm and dry.

For more weather updates, follow me on Twitter: @akracki and @TheWxCentre.

Andrew

April 26 Potentially Significant Severe Weather Event

A potentially significant severe weather event continues to show up in model guidance for Tuesday, April 26th over the Central and Southern Plains.

Storm Prediction Center

The Storm Prediction Center's long range outlook for Tuesday places a 30% chance of severe weather within 25 miles of a given point across northern Texas, much of central and eastern Oklahoma, and southern Kansas. A 15% chance of severe weather within 25 miles of a given point is situated from central Texas north into Oklahoma (outside of the panhandle), much of central and eastern Kansas, and extreme western portions of Missouri and Arkansas.

CoD

This event will begin taking shape on Monday, April 25th, when the storm system that will act to ignite this potential outbreak makes landfall on the West Coast. 500-millibar speed max near 100 knots barreling into southern California and western Arizona will be associated with a vorticity max rounding the base of the main trough, whose energy will be extended from northern California offshore Oregon and Washington by Monday morning. This speed max will shunt the aforementioned vorticity max east, where it will then round the lee side of the trough and act to pull the main trough northward by Tuesday morning.

The main energy associated with the trough will continue moving southeast into the Southwest U.S. and strengthen as another speed max rounds the base of the trough, this time making it negatively-tilted. Recall that a negatively-tilted trough means the highest wind speeds are 'pointing' the trough in a southeast direction. This indicates the trough is mature, and is optimal for severe weather.

CoD

As mentioned above, by Tuesday night at 7 PM Central Time, we see a speed max just breaching 70 knots traversing the Central Plains, indicating the trough is now negatively-tilted and at max strength. The trough here is in prime positioning for a severe weather event to unfold across the Central U.S. in the evening and overnight hours on Tuesday, and will set the stage for continued opportunities for severe weather as the week continues.

CoD

Strong moisture flow prior to the trough's entrance to the region will allow surface dewpoints to surge into the 70s by Tuesday evening, as the above graphic shows. As such, instability on the order of 4000 j/kg to 5000 j/kg is anticipated across Oklahoma into northern Texas, approximately in line with the 30% delineation in the Storm Prediction Center graphic at the beginning of this post. This ample moisture will allow for cloud bases to drop below 1000 meters- soundings from points across central and northern Oklahoma suggest cloud bases (also known as LCLs) will be around 600-800 meters off the ground, which can be conducive for tornado development. 800 meters off the ground is a little high for prime tornado formation conditions, but ample instability may act to offset a portion of this.

CoD

Lastly, supercell composite forecasts off the 18z GFS suggest the environment most conducive for supercell formation will be from southern Kansas across central Oklahoma, just into north Texas, again in line mainly with that 30% severe threat corridor. Although supercell composite values are high in central Kansas, the best axis of instability will be displaced in extreme southern Kansas and central Oklahoma, and it is this latter region where the best potential for severe weather lies. In this region, taking current model guidance at face value, all hazards of severe weather would be possible. This would include a threat for strong, possibly long-track tornadoes, very large hail, and damaging winds.

To Summarize:

- A severe weather event is being forecasted for April 26th.
- Substantial uncertainty exists regarding storm initiation, threat level, and placement of best environment, among other things.
- As such, confidence is low with this event.
- Going by current model guidance, a potentially dangerous severe weather situation could evolve across northern Texas, Oklahoma, and southern Kansas on April 26th.

Andrew

Severe Weather Discussion for March 30, Posted March 30

A severe weather event is forecasted to unfold across the Plains and Mississippi Valley today, March 30th.

Prognosis…
           Large upper level low currently stationed across the Western US will provoke a potential severe weather event in the Plains and lower Mississippi Valley today. Situation is complicated as convection later in the day will be heavily influenced by morning convection, residual cloud cover, as well as dryline placement later in the day.

Overnight…
               Stout upper level low centered across Nevada and Utah will begin to flatten as energy currently located in the Dakotas is ushered northeastward in pace with the jet stream, which will be arching north of mid-level ridging currently stationed in the lower Mississippi Valley. This energy in the Dakotas will be drawn towards another system in south-central Canada, resulting in a band of elevated vorticity extending from Canada back to Nebraska at 12z today. Further, the vorticity max currently in southern California, extending in a semicircle offshore the West Coast into Oregon, will keep the upper level low positively tilted and aid in this flattening-out.
    Moisture flow will continue to amplify across the Southern Plains, with water vapor imagery showing the subtropical jet feeding a moisture plume into the region. As such, cloud development occurred across the area earlier Tuesday and continues to persist at the time of posting. To this extent, continued moistening of the planetary boundary layer through isentropic ascent, notable 850-millibar moisture plume advection, and substantial low-level warm air advection will result in areas of drizzle to showers overnight, particularly across Texas and Oklahoma. 3/30 00z KOUN sounding showed strong temperature inversion between 800-millibar and 700-millibar levels, and I suspect this inversion will be strong enough to suppress anything more than broad, generally-weak precipitation. Surface plots at 0700z show areas of mist and light rain across central and eastern Texas, in line with favored model guidance. Model guidance troubles will be discussed more in depth in the following sections. Uncertainty with this particular subject of morning precipitation will feed into the morning and afternoon discussions below.

Early Morning to Noontime…
         Convection-allowing model guidance has been promoting the formation of showers and thunderstorms along the Red River, most notably in southeast Oklahoma in the mid-morning hours today, following expected overnight precipitation and continued moistening of the lower levels. Not completely convinced we see hail threats as advertised by some, including the threat of up to tennis ball-sized hail as per NWS OUN in their hazardous weather outlook posted yesterday. However, as the atmosphere continues to become more conducive to severe weather through continued warm air and moisture advection overnight, will not rule out severe weather tomorrow morning, especially given expected values of elevated instability that may favor stronger storms. I do believe hail is a primary threat with these storms, especially in their elevated nature, but again, I am not completely sold on the idea of severe and/or significant hail.
         Primary point of this timeframe discussion is to what extent precipitation is ongoing by and after 12z. Model guidance has been hinting at potential convection across portions of Oklahoma, especially south and east in the morning hours, but has also left the door open for weaker, more broad precipitation in central and northern Oklahoma. Regardless, expect overcast cloud cover given continued lower-level warm air and moisture advection, as well as taking into account 3/30 00z soundings across Texas and the western Gulf Coast. I am not taking the HRRR model into account for this discussion, as although it is currently handling surface precipitation (or lack thereof) rather well, it brings surface temperatures across Oklahoma and Texas into the 80s and 90s, well above current and more-realistic temperature forecasts introduced by the NWS. Additionally, am not using the NCAR ensemble, as all members have weak, broad, and scattered precipitation at initialization (3/30 00z) across Texas, which was proven to not be the case when compared with 3/30 00z radar. In terms of morning precipitation, am currently favoring 00z 4km WRF-NMM model, as well as SPC SREF system, as both models handle initialization conditions at the surface with regards to precipitation coverage well, and perform well with expected high temperatures later in the day. Given high model uncertainty with this event, however, making these two forecasts ‘favored’ is not exactly as affirmative a word as it is made out to be.
          In all, expectation is for precipitation, possibly convective in nature, to be ongoing in portions of Texas and Oklahoma by daybreak today and in the morning hours in general. Most favored region for this activity, as promoted by model guidance and SPC probabilistic thunderstorm outlooks, is southeast Oklahoma into northeast Texas, east into Arkansas. Weaker and more broad precipitation also possible for central Oklahoma, where uncertainty is greatest.

Afternoon and evening…
        Remarkably complex situation will unfold for the afternoon and evening hours. It has been noted that the upper level low in question has been progressing east more slowly than anticipated yesterday, and this has been feeding into a further-west trend with the dry line positioning later on in the afternoon today. Case in point, 4.0km WRF-NMM guidance shows convective initiation in the afternoon hours along the dry line, which is projected to be planted almost immediately east of the Oklahoma panhandle in due north-south orientation. This is the most westward solution I’ve seen, and while it has handled overnight precipitation well so far as of this posting, and the upper level low has been progressing slower than thought, the far westward positioning of this dry line is something to monitor. To speak more frankly, this solution is suspect, although this model still remains among favored guidance as of posting time. 3/29 21z SPC SREF takes a nice middle ground between this westward solution and further-east guidance, placing the dryline approximately along a north-south line of Lawton, Oklahoma to Burlington, Oklahoma by the afternoon hours. As such, thunderstorm initiation along the dryline would threaten the Oklahoma City metro area, and eventually may threaten the Tulsa region.
        Concerns have been brought up frequently in discussions around the lack of convective initiation on the dry line in some model guidance, despite the presence of an uncapped, supercell-favorable storm environment. This would be the result of a lack of trigger mechanism, it appears, as some guidance shows weak convergence along the dry line. I am personally opposed to this solution, and while it may very well pan out, I am more pressed to believe favored guidance, which does initiate convection along the dry line in the afternoon hours.
In the event convection does initiate along the dry line, it is expected that thunderstorms would quickly become supercellular, with all modes of severe weather possible. The new SPC Day 1 outlook has placed central and southern Oklahoma in the Enhanced Risk of severe weather, primarily for potentially significantly large hail, in addition to damaging winds and a non-zero, yet relatively low tornado threat. Although it still retains unrealistically-high surface temperatures tomorrow afternoon, new runs of the HRRR model continue to gradually increase the coverage and intensity of convective initiation along the dry line in central Oklahoma. Initiation here is along the Lawton-Burlington line previously mentioned, but I am not willing to put stock into the HRRR currently given its unrealistic surface temperatures.
       Outside of Oklahoma, best tornado dynamics outlined by SPC Day 1 projection and SPC SREF guidance will be placed in Louisiana/Arkansas/Texas area, where a 10% hatched delineation has been posted in the Day 1 outlook. Despite this, as mentioned previously, non-zero tornado threats will exist in Texas and Oklahoma in addition to these more favorable dynamics.

        To summarize, potential severe weather event is on the table today for the Southern Plains into the lower Mississippi Valley, with high uncertainty still included in the forecast. Broad, weak precipitation expected to develop and move into Texas and Oklahoma overnight into the morning hours will likely temper a full-on severe weather outbreak for the Plains, but afternoon clearing and favorable dry line positioning may allow the formation for strong to severe storms in the afternoon across central and eastern Oklahoma. Widespread convection is expected for the eastern Texas / northern Louisiana / Arkansas region, where tornado dynamics will be best. Large hail will be a day-long threat for the Oklahoma area, particularly with a round of thunderstorms in the morning that may form in the midst of the weak, broad precipitation to produce hail, and then again in the afternoon when potentially-supercellular development occurs along the dry line.