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FXUS02 KWBC 220618

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
218 AM EDT Wed May 22 2019

Valid 12Z Sat May 25 2019 - 12Z Wed May 29 2019

...Record/Dangerous Heat for the Southeast this weekend into next

...Heavy rainfall possible for portions of the central Plains/Corn


Rather stagnant but highly amplified upper level pattern will
continue to be dominated by deep troughing in the west and strong
ridging in the Southeast. A closed low will sink through/along
coastal California this weekend to bring another round of
precipitation to the west under a chilly airmass. The
Northeast/Mid-Atlantic will lie in the confluence between the
northern and southern stream as a wavy boundary oscillates
initially over the Carolinas lifts northward as a warm front
before a cold front pushes out of the Midwest into the
Mid-Atlantic late Sunday. The pattern favors hot/dry conditions in
the Southeast with record temperatures near and just over 100F. In
between, broad southwesterly flow aloft and southerly flow at the
surface will favor rounds of convection and locally heavy rainfall
with multi-inch totals over the period especially over the central
Plains into the Corn Belt.

...Guidance Evaluation and Preferences...

Latest guidance proved to be a good consensus/starting point for
the forecast, though the 12Z UKMET was quicker than the
GFS/ECMWF/Canadian with the upper low in the west Sat. In general,
a blend of the GFS/ECMWF added some detail to the forecast through
about next Mon/D5 given some continued differences in timing/track
of the northern stream (especially across Canada due to
difficulties resolving anticyclonic wave breaking east of Alaska).
Next week, ECMWF and its ensembles were slower to eject the upper
low out of the west compared to the GFS/GEFS mean while the
Canadian was in between. Favored only the ECMWF/ECMWF ensemble
mean by next Tue/Wed as the trend has been for a slower speed and
since the GFS/GEFS typically is too quick by ~6-7 days out with
initially amplified features. However, trough may eject in pieces
rather than wholesale per some ensemble members. Ensembles favor a
more positively-tilted trough by next Wed with a flatter ridge in
the Southeast and western Canada.

...Weather Highlights and Hazards...

Sustained heat will become increasingly intense in the Southeast
into next week with widespread temperatures in the upper 90s and a
modest area of 100-103F which will challenge monthly records.
Overnight lows may slowly struggle to fall to 70F which will pose
additional health stresses due to the early timing of this heat,
more typical of June/July. Heat indices over 100F are likely for
many areas under the upper high with no chance of rain due to the
stable atmosphere. Temperatures will trend toward average farther
north along the east coast.

The west, by contrast, will see well-below average temperatures by
about 15-25F with abundant cloud cover and modest rain with some
high elevation snow. Some light rain is possible over most of
California (though focused over the Sierra) even down to southern
areas but probably not into the deserts. To the east, temperatures
around the wavy boundary from the central Plains eastward will
rise/fall as the front lifts northward or slips southward. This
will also provide a focus for rainfall and some convection. Rounds
of rainfall with locally heavy amounts may add up to several
inches over the period, some of which may fall within a short
amount of time over areas that have seen quite a bit of rainfall
recently. Greatest flooding/flash flooding risk during the period
seems to be centered over central/eastern Kansas northeastward
through Iowa into Wisconsin by next Wed.


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

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