Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
312 PM EDT Mon May 10 2021
Valid 12Z Thu May 13 2021 - 12Z Mon May 17 2021
1800 UTC update...
The models and ensembles continue to show fairly good agreement
(especially the first half of the period) with the overall large
scale pattern, but larger differences in the details. There is not
a whole lot to note with the new 00z/06z guidance, with the same
forecast concerns from the overnight shift as noted below. Given
this, maintained pretty close WPC continuity with no major changes
to the forecast needed. The updated guidance favored a blend
towards the deterministic ECMWF/GFS/CMC/UKMET days 3-5, with more
weight on the ensemble means/ECMWF days 6-7. See previous
discussion copied below for the details.
Previous discussion issued at 0655 UTC...
Latest guidance continues to show moderate progression to flow
aloft across the lower 48 and vicinity. Aside from fairly minor
details, there is reasonable consensus for a broad mean trough to
cross the eastern U.S. and emerge into the Atlantic while a
trailing ridge over the West as of Thursday should reach into the
Mississippi Valley by next Monday. There has been more spread and
variability for details of upstream Pacific energy that should
amplify into an upper trough that reaches/crosses the West during
the weekend, as well as for details of flow behind this feature.
Aside from a front dropping through the Gulf and Florida Peninsula
late this week, waves/fronts will primarily affect the
western/central U.S. as high pressure prevails over the East.
Thus the best focus for precipitation during the period will
likely extend from the northern Rockies through a majority of the
Plains and into the Middle/Upper Mississippi Valley. Contrasting
temperatures on Thursday, with parts of the West quite warm and
the South/East much cooler than normal, should steadily gravitate
toward climatology with time.
...Guidance Evaluation and Preferences...
With current model runs still showing minimal surface response to
the upper trough crossing the East aside from waviness well
offshore, the dominant forecast difficulties involve the shortwave
energy expected to move into the West during the weekend and
upstream flow that will affect the pattern behind this energy.
After a couple days of increased spread for where/when features
could try to separate, 12Z/18Z model/ensemble runs (and followed
up in the new 00Z runs) have returned to some variation of 2-3
days ago when there was better clustering toward the idea that any
separation that occurs would wait until energy reaches near the
West Coast. There are still significant differences for what
individual pieces of energy will do though, and predictability is
low due to their small scale. A blended approach (more models
than means into day 5 Saturday followed by a more even mix by day
7 Monday) continues to provide the best way to navigate the spread
and variability that the models/ensembles have exhibited so far.
Farther west, recent GFS runs trend out of phase with North
Pacific flow from the weekend onward--a result of western Pacific
flow separation which other guidance does not depict. In contrast
to the flat flow to ridging that the GFS shows near the West Coast
by next Monday, ECMWF/CMC runs and their ensemble means have been
fairly consistent so far in bringing another trough toward the
West--albeit with detail and amplitude differences. Latest GEFS
means fall between these two extremes, with a weaker and delayed
trough approaching the West. Current preference is to tilt the
forecast somewhat more in the direction of the ECMWF cluster
relative to the GFS/GEFS.
Expect a gradual increase in coverage and amounts for rain and
thunderstorms over the central U.S. and northern Rockies from late
this week into early next week. Initial activity will be focused
mostly by weak northern stream shortwave impulses and there should
be a relative lack of deep moisture. Then Gulf moisture should
begin to contribute more and interact with one or more
waves/fronts as eastern U.S. high pressure nears the East Coast.
With uncertainty in the specifics, shortwave energy emerging from
the West late in the period may provide another ingredient in the
rainfall recipe. Currently expect the highest five-day totals
over portions of the Central/Southern Plains and Middle/Upper
Mississippi Valley. Some precipitation could be in the form of
snow over highest elevations of the northern Rockies. Any
precipitation farther west should be fairly light and scattered.
The upper trough crossing the East late this week may produce some
areas of mostly light rainfall over central/northern latitudes,
with greater coverage during the latter half of each day.
Locations near the Southeast coast and over the Florida Peninsula
could see somewhat more organized rainfall for a time before a
drier trend, as a wavy front initially over the Gulf and Florida
Peninsula pushes southward.
On Thursday the southern tier will see well below normal highs
with minus 10-20F anomalies over parts of the Southeast and minus
10-15F anomalies over parts of Texas. Some locations in the
Southeast could challenge daily records for cool highs. Less
extreme below normal readings should prevail late this week over
the remaining majority of the central/eastern U.S. Meanwhile
temperatures are likely to be up to 10-20F above normal over areas
from Oregon/California into the Great Basin late this week with
more moderate anomalies stretching into the central Rockies/High
Plains late this week into the weekend. During the weekend upper
troughing that moves into the West should bring temperatures back
down toward normal across the region from west to east. The
moderating trends for both cool and warm areas from late this week
may yield highs within a few degrees on either side of normal over
much of the lower 48 by next Monday.
- Heavy rain across portions of the central and southern Plains,
Mon, May 17.
- Flooding possible across portions of eastern Texas to the
Louisiana border, eastern Kansas, as well as northern Ohio.
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: