Prognostic Meteorological Discussion
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FXUS02 KWBC 231557

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Wed May 23 2018

Valid 12Z Sat May 26 2018 - 12Z Wed May 30 2018

...Potential multi-day heavy rainfall event possible for the lower
Mississippi valley and Southeast...


The flow pattern across North America will become a bit more
blocky during the medium range, with persistent and relatively
slow-propagating height anomalies. Negative height anomalies are
expected to persist across northeast Canada and Greenland, with
strong positive height anomalies across northwestern Europe.
Teleconnections associated with these features are a bit
contradictory, however the stronger anomaly over Europe supports
the idea of persistent subtropical ridging along the U.S East
Coast, and across western U.S./Canada, with potential for Pacific
shortwave energy to undercut the ridge - which fits the dominant
pattern through the medium range. An area of low pressure expected
to be in place across the Gulf of Mexico on day 3 (Sat) will
struggle to find a quick path northward given the ridging
north/east of the system, keeping the westerlies confined
relatively far to the north. A cutoff upper low crossing the Great
Basin on Sat is also expected to slowly drift east-northeast
across the Rockies through early next week before likely becoming
absorbed in the westerlies and moving east across the northern
plains. Finally, additional shortwave energy is likely to approach
the West Coast by next Tue-Wed.

...Guidance and Predictability Assessment...

With respect to the area of low pressure crossing the Gulf of
Mexico during the period, models/ensembles show two distinct
camps. The 00Z ECMWF/CMC along with the ECENS/CMCE means take the
system on a northwesterly track, nearing the mouth of the
Mississippi River by Sat night/Sun. The GFS/GEFS on the other hand
take the system more northward or even slightly east of north,
crossing the Florida Peninsula and the southeastern states. At
this time, given the upper ridging prevalent north and east of the
system, WPC preferred the more northwesterly ECMWF/CMC solutions.
Additionally, ensemble guidance has shown a broad trend over the
past couple cycles toward a slower northward progression of the
system by days 6-7 (next Tue-Wed). As a result this forecast
update reflected a slight trend in that direction, keeping the
system a bit farther south by that time, and showing a slow
westward propagation along the Gulf Coast. Given the divergent
model ensemble camps, forecast confidence at this time is moderate
at best.

Farther north, model spread is lower, and forecast confidence is a
bit higher. The GFS/ECMWF/CMC all handled the evolution of the
western U.S. upper low during days 3-5 similarly. Some differences
emerge by days 6-7, with the GFS/ECMWF bringing the system under
the influence of the westerlies and moving it east into the
northern plains, while the CMC continued to hang much of the
energy back across interior West (while also bringing part of the
energy east into the northern plains). Additional shortwave energy
skirting the northern Great Lakes Sat-Sun is also handled
reasonably well by the GFS/ECMWF/CMC. Finally, additional
shortwave energy looks to approach the West Coast by the middle of
next week. Models show general agreement on the existence of this
feature, but run-to-run variations exist with respect to the exact
character of the feature (e.g. a full-latitude upper trough/upper
low or a more progressive open wave).

Given these considerations the WPC forecast was initially based on
a multi-model deterministic blend including the 00Z ECMWF/CMC/GFS
(with a solution more toward the ECMWF/CMC for the Gulf of Mexico
system) during days 3-5. During days 6-7, weighing of ensemble
means (00Z ECENS/NAEFS) was boosted to account for increased

...Weather/Threats Highlights...

Regardless of the final track that low pressure in the Gulf takes,
portions of the Gulf Coast and Southeast will see the potential
for a multi-day heavy rainfall event beginning this weekend and
persisting into the middle of next week. Given the preferred
ECMWF/CMC solutions, much of the Gulf Coast, lower Mississippi
valley, and Southeast would see the potential for heavy rains
spread across several days, with the heaviest rains potentially
falling near the mouth of the Mississippi River. If the GFS
solution were to verify, heavy rains would be much farther east,
with a significantly reduced heavy rain threat for much of the
Gulf Coast. A farther east scenario would also potentially direct
the transport of higher moisture content air into the
Mid-Atlantic, where interaction with a weak surface front would
potentially result in heavy rainfall (as shown by the GEFS mean).
Elsewhere, the upper low crossing the West could produce locally
heavy rains across the Great Basin or portions of the northern
Rockies, and the shortwave skirting the Great Lakes/Northeast
Sat-Mon could produce scattered showers and thunderstorms with
locally heavy rain possible.

Temperatures are expected to be well above average across much of
the central U.S. during the medium range. High temperature
anomalies of +10 to +20 deg F are forecast Sat-Mon across much of
the Plains, with the greatest anomalies from the northern plains
to the Upper Midwest. A number of record high temperatures could
be in jeopardy.


WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:

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