Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue May 11 2021
Valid 12Z Fri May 14 2021 - 12Z Tue May 18 2021
...Heavy rainfall possible next Monday and Tuesday for parts of
the Southern Plains into the Lower Mississippi Valley...
The best clustering of guidance shows an evolution toward some
degree of mean troughing over the West while mean ridging
progresses across the central U.S. into the East behind a broad
upper trough that departs from the East Coast this weekend. There
is a small minority that trends out of phase with this idea by
late in the period though. As the most likely pattern develops,
expect a gradual increase in coverage and intensity of rainfall
over the central U.S. with time. Some rain/thunderstorms could
eventually extend into parts of the East while parts of the
Rockies may see some precipitation as well. Contrasting
temperature anomalies late this week with warmth over the West and
cooler than normal readings to the east of the Rockies should
generally moderate with time, leaving many areas fairly close to
normal by early next week.
...Guidance Evaluation and Preferences...
The 00z/06z guidance continues to show decent agreement with
either an amplified shortwave/closed low over California around
early Friday. This energy would then eject eastward into the
southern Rockies or High Plains by early next Tuesday. As was seen
24 hours ago, significant divergences develop for the large scale
pattern by late in the period. Although the GFS appears to have
fixed some of its prior issues with its western/northern Pacific
forecast, by the latter half of the period it still becomes slower
and more amplified with flow from the eastern Pacific into North
America, leading to showing a ridge over the West by early next
week while developing a fairly deep closed low over or near the
Great Lakes. In contrast, most remaining guidance shows some
degree of cyclonic flow arriving into the Northwest U.S. while
above normal heights/ridging prevail over the East. GEFS mean
reflects a compromise, being a little slower than the ECMWF/CMC
and their means with troughing that heads toward the Northwest
while depicting only a shortwave reaching the Great Lakes and
vicinity. WPC favored a blend close to previous shift continuity,
leaning on the deterministic runs days 3-5, with increasing weight
towards the ensemble means/ECMWF late period.
Expect central U.S. rainfall to focus initially over parts of the
Central Plains and Midwest ahead and north of a wavy warm front.
Some of the activity over the Midwest could eventually reach into
the Ohio Valley/eastern states as the focusing warm front extends
farther eastward. Rain and thunderstorms should also expand
southward and become heavier as return flow behind eastern U.S.
high pressure brings in more Gulf moisture and western U.S.
shortwave energy/possible upper low approaches the Plains. The
guidance shows a signal for heavy rainfall, especially over parts
of the Southern Plains into the Lower Mississippi Valley next
Monday/Tuesday. Parts of this area has seen much above normal
precipitation over the past several weeks, which may increase
flooding concerns. Across the Rockies/High Plains, there may be
some enhancement of precipitation near the central Rockies/High
Plains late in the period depending on the evolution of the
feature emerging from the West. In most cases any precipitation to
the west of the Rockies should be fairly light and scattered.
Meanwhile, the upper trough moving through the East late this week
into the weekend should promote areas of mostly light rainfall
from the Appalachians into the Northeast.
Well above normal temperatures centered over the Great Basin and
vicinity will extend into Friday with plus 10-20F anomalies for
highs. Then an upper trough/potential closed low should promote a
cooling trend over the southern half or so of the West, with parts
of the far southwestern U.S. possibly seeing a day or two with
slightly below normal highs. A trailing upper ridge may promote a
brief rebound over the Interior Northwest on Sunday followed by
cooling trend and southward suppression of remaining above normal
readings as cyclonic flow aloft approaches/arrives. Over the
eastern half of the country expect below normal temperatures to
prevail over many areas late this week. Locations near the
southeastern coast could see highs up to 10F or so below normal on
Friday. Then a moderating trend will likely yield near normal
highs and slightly above normal lows over the eastern half of the
lower 48 by early next week.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
- Heavy rain across portions of the Central Plains, the Lower
Mississippi Valley, the Southern
Plains, the Middle Mississippi Valley, the Great Lakes, and the
Ohio Valley, Sat-Mon, May 15-May 17.
- Heavy rain across portions of the Middle Mississippi Valley, the
Lower Mississippi Valley, and
the Southern Plains, Mon-Tue, May 17-May 18.
- Flooding possible across portions of the Great Lakes, the Lower
Mississippi Valley, the Southern
Plains, and the Ohio Valley.
- Flooding occurring or imminent across portions of the Great
Lakes, the Lower Mississippi Valley,
the Southern Plains, and the Ohio Valley.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: