Long Range Chatter - March 30, 2013

Temperature anomalies from the last week show the lack of spring continuing throughout nearly every region of the nation, save the Southwest. Temperature anomalies, shown in Kelvin, dropped as low as 10 degrees below normal in the Dakotas, with even the Gulf of Mexico experiencing some of this cool weather. This cold weather has been provoked by prominent high pressure over the Arctic Circle and Greenland, as this reanalysis image from March 25th shows:

500mb height anomalies are placed on the left, where warm colors depict high pressure
and cool colors indicate low pressure. Mean sea level pressure contour lines, as well as cloud
cover are shown on the right half of the image.

In response to this extreme high pressure over the Arctic and Greenland, low pressure has been provoked to develop over the central and eastern US, more-so over the latter region. As long as we see that high pressure area over the upper latitudes, chances are more than likely that the tendency for cooler weather will continue to fall upon the central and eastern US. With the cold weather, we also expect a tendency for low pressure in very similar regions, so storm systems moving into the eastern US could be more commonplace than instances of above-normal temperatures.

European ensembles predict that this high pressure will remain stagnant over the Arctic and Greenland from now until at least 10 days away. Consequential low pressure will also stay in place north of the Canadian Maritimes and into eastern Canada. New England will also be affected by this relatively stagnant body of low pressure, and these effects may range from below normal temperatures to unusually cloudy days (both of which I consider likely). High pressure attempting to develop over the Western US will enhance cold weather potential south of Canada, possibly encompassing millions east of the Mississippi.

European model temperature anomalies 5000 feet above ground display this cold weather trend, clearer than day in the Northeast. Below normal temperature anomalies resonate well into Canada, sideswiping the Great Lakes and Upper Midwest. Warmer than normal temperatures will assist the skiing season in coming to a close in regions where resorts remain open. This forecast, valid for forecast days 6-10, is pretty likely to turn out when you look at what has been experienced and comparing it to what is being forecast. High pressure will remain favorably positioned over the Arctic to allow cold weather to hit regions of the US previously hit by the cold, and also spread eastward.

Summary
Next 10 days look to remain below normal in temperatures for the central and eastern regions of the United States. Warmer than normal temperatures should persist in the Western US. I feel that best chances for storm systems will be in the northern Gulf Coast (TN/AL/MS/GA) and north, along the Eastern Seaboard and into the New England area. Probably will not see any significant Nor'easters in this timeframe, although a wayward storm system well offshore may allow coastal cities to get a quick bout of precipitation.

Andrew

April 5-6 Potential Coastal Wintry Storm

Model guidance remains iffy on the prospect of a potentially wintry storm system between April 5-6 in New England.

Discussion: Previous model guidance indicated this storm would be a mostly rain event, but new model guidance has this storm system producing wintry precipitation in southern Maine and a few other coastal states in the wake of this system. The American model, pictured above, has the system stay in above-freezing temperature territory for the heavy majority of its lifespan, hitting coastal towns with a rain event. As is the case with springtime storms, temperatures are much more marginal with this storm than typical winter storms. There is potential that there will be no snow from this system and it could be all rain. Logical forecast to go with right now is a rain event with weak backend snowfall. Heaviest snowfall (if any) would affect Maine, the state furthest north and thus furthest into the colder temperatures. Light lake effect precipitation is likely to commence when the storm moves off, but as winter draws to a close, we find the potential for massive lake effect snow events rapidly decreasing. American model has snowfall amounts staying at or below 2 inches for select coastal areas, whose locations are to be determined.

Andrew

April 10-12 Potential Storm

American GFS model MSLP anomaly forecast for 10 days

European ECMWF model 500mb anomaly forecast for 10 days

Medium-range model guidance is in unusually good agreement on a storm system digging into the Southwest US as a closed low before making its way east and affecting areas east of the Front Range. Below normal anomalies for the model's respective fields shown above depict the storm system, while the warmer colors in both images indicate high pressure. European model is considerably further north with this storm system over the American GFS. Because the European model only forecasts out to 10 days, we cannot see how far in the future the European takes it. However, the American model DOES go through the full timeframe of this storm, and we will examine its forecast a little later on.

General atmospheric flow in both images is highly meridional, and the lack of a positive tilt (highest vorticity pointing towards the southwest) means that this system should quickly occlude and shoot north into the Plains towards Canada. When this occlusion happens, an accompanying cold front is likely to begin forcing the massive high pressure system out. Based on temperature forecasts from American model and European ensemble system, temperatures surpassing 60 would be common in and south of the Great Lakes. This could set the stage for a severe weather event. European model itself has cities like Chicago and Des Moines flirting with the upper 70's as the high pressure system takes over as illustrated in the above images, while the American model has those cities closer to the 60's.

Precipitation forecast on April 12th suggests the suspicions of a severe weather event may verify, as a heavy precipitation event looks to impact the southern Plains, likely encompassing stronger storms in the typical cold front's linear storm formation. Time will tell if this linear formation ends up being a squall line or just pockets of heavy rain. Lighter amounts also exist further to the north, stretching as far north as far northern Wisconsin. Examination of severe weather parameters for this timeframe reveal rather high deep level shear conducive for at least marginally strong thunderstorms. Lack of widespread instability across southern Plains is something to watch, although elevated instability immediately onshore of the Gulf Coast would allow for some solidly strong thunderstorms to thrive.

Andrew

April 5-6 Potential Storm

There is potential for a coastal storm in the Northeast from April 5-6.

Fresh model data indicates a storm system will form in the Southeast and manifest itself to move northeast, just offshore of New England. The system will not be close enough to provide an opportunity for snow, and the presence of warm air along coastal cities means any snow potential will be dashed there by a chilly rain. Lake effect snowfall may be possible in western New York, but this looks to be a rather marginal opportunity for any accumulating snow. Best chances for accumulation reside in western New York as a result of the lake effect snow, and those accumulations may be very light. There remains time for this situation to change, but right now I'm seeing a non-event as far as snowstorm potential goes.

Andrew

Summer Warmth On The Way

The latest run of a member of the European ensemble set has summertime warmth flooding the nation just 10 days from now.

The image above is from the European ensemble control run, which is a member of the ensemble set that has its parameters unchanged. As you may recall, ensemble members are forecasts from the model itself, but their starting parameters are slightly changed in one way or another. The control run does not have its parameters changed. This forecast, valid for the evening of April 8th, has a massive warm-up across much of the nation, with every state east of the Colorado Front Range (except a few select New England states) reaching full-on spring or summertime warmth.

Cities like Houston, Orlando and Atlanta would have temperatures very similar to those in Chicago, St. Louis and Detroit. The massive warm-up would not reach northern ND, MN, WI or the Upper Peninsula of Michigan, although a forecast 240 hours out is not nearly written in stone.

The unusually warm temperatures following this unusually cold start to spring would most likely be a byproduct of an intense ridge of high pressure sitting across the Great Lakes, Midwest and Plains. Latest indications are that this warmth would be wiped out a few days later as the storm system you see in the Southwest moves on through the nation, but the incredible expanse of the high pressure system means the warmth would not go down without a fight. Time will most certainly tell if this warmth comes to fruition, but this forecast certainly warms the heart of those frowning upon the cool and snowy start to spring.

Andrew