Sunday, May 26, 2019

June 1-6 Potential Warming Trend

Model guidance is beginning to fixate on a broader trend away from cooler than normal temperatures in the central and east U.S. into a more seasonal, if not above-normal temperature pattern. Click on any image to enlarge it.

Forecasted 500 millibar geopotential height anomalies per the GEFS, valid 1pm June 1st.
Source: Tropical Tidbits
By June 1st, the GFS ensembles (GEFS) anticipate somewhat of a pattern change, albeit only to a modest degree, from the troughs continually dropping into the West, bowling east through the Plains and then shifting northeast through the Great Lakes. The ensembles here anticipate a trough to again drop into the Southwest, but this time mild ridging is projected to build into the Pacific Northwest as an upper level low migrates eastward from the Bering Sea into the Gulf of Alaska. Note that the ridge in the Pacific Northwest and southwestern portion of Canada will be tempered as the Pacific jet stream carves its way through west-central Canada, as opposed to carrying the trough down into the Southwest U.S.

Typical atmospheric pattern in a Rex Block pattern.
Source: National Weather Service
The result of this marginal shift in pattern is a Rex Block-like formation over the Western U.S. The image above shows a textbook Rex Block, where a ridge builds to force the jet stream northward, but an upper level low is positioned almost directly south of the ridge. The result downstream of the Rex Block is zonal flow, as shown by the almost-directly west-to-east isohypse alignment.

We see almost that same pattern in the GEFS forecast at the top of this post, with a ridge building into parts of the Pacific Northwest and southwest Canada, a trough to the south of that ridge, and zonal flow across the rest of the country as a result. Further, also seen in the textbook Rex Block image, we still see that stubborn upper level low positioned in southern Canada, the same feature responsible for repeated shots of cooler than normal air for swaths of the northern U.S. as of late. For the record, continued ridging over the North Pole affirms these bouts of cooler than normal weather will remain possible for the northern U.S.

Forecasted 850 millibar temperature anomalies as of 7am, June 4th.
Source: Tropical Tidbits
While the warming trend begins on June 1st as that zonal flow takes hold, the strongest above-normal temperature anomalies just a few thousand feet off the ground emerge on June 4th, with anomalies on a magnitude of 10 degrees Celsius above normal or higher forecasted by the GFS ensembles over parts of Oklahoma, Kansas, Nebraska and South Dakota. The entire Central U.S., parts of the Southeast and much of the Rockies look to be encompassed in these above-normal temperature anomalies before more seasonal weather begins to filter into the country's midsection.

After June 6th, model guidance begins to become more uncertain as to the direction of the pattern, but early indications are warmer-than-normal temperatures may sustain themselves in the Western U.S., leading to cooler than normal conditions for the eastern third of the country. I discuss the broad pattern outlook in a special long range outlook post here from last week, but already it appears that my early-June forecast may miss the mark for failing to catch this nascent warming trend.

To summarize:
- A warming trend is forecasted for portions of the Central and East U.S. beginning June 1st and continuing through June 6th.
- The warmest conditions look to be centered on the Plains, but temperatures gaining to at least seasonal levels appears likely from the Front Range in Colorado out into the Ohio Valley.
- Forecasts diverge beyond June 6th, but warmer than normal conditions may persist in the West while cooler conditions take over in the East.