Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed May 12 2021

Valid 12Z Wed May 12 2021 - 12Z Fri May 14 2021

...Showers and thunderstorms will persist over the Gulf Coast states
through tonight and lessen by tomorrow...

...Below normal temperatures are expected east of the Rockies while the
western U.S. warms...

A frontal boundary that has been meandering along the southeastern U.S.
for a few days will finally start to push offshore by tomorrow and dry out
the region for the most part. However, through tonight, additional showers
and thunderstorms are possible across the Gulf Coast states and into the
Carolinas. Flash floods remain possible, especially since the Gulf Coast
region has seen wet conditions over the past several weeks. Additionally,
portions of the Southeast could see strong to severe thunderstorms, with
strong winds and hail possible in southeastern Alabama, southern Georgia,
and northern Florida.

Elsewhere, scattered showers and thunderstorms are possible for the
Northern and Central Plains over the next couple of days. On Thursday,
some storms could have strong winds associated with them in the Central
Plains. Some light showers are also possible in the Northeast today,
mainly in the form of rain, but some light snow is also possible this
morning in the highest elevations.

Temperatures are forecast to be below normal generally east of the
Continental Divide, though Florida should remain warmer than normal today
before the cold frontal passage. Cold morning lows will once again cause
Freeze Warnings and Frost Advisories across the the Central Plains, Great
Lakes, Ohio Valley, and into the Northeast. Cool high temperatures could
set daily records across the south-central to southeastern U.S. today,
with highs only in the 50s and 60s. Meanwhile, temperatures are forecast
to be 10 to 20 degrees above normal in the West, with temperatures in the
90s in some areas and rising to 100 degrees in the Desert Southwest.

Tate
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Wed May 12 2021

Valid 12Z Sat May 15 2021 - 12Z Wed May 19 2021

...Heavy rainfall possible over parts of the Southern Plains into
the Mississippi Valley Monday-Wednesday...

...Overview...

Thanks to recent trends in GFS runs toward the ongoing majority
consensus, today's guidance shows better agreement with the large
scale evolution during the period. Within a mean trough aloft
forecast to be established over the West, a leading trough/upper
low progressing from California through the Southwest or Great
Basin and then ejecting into the Plains should be followed by an
upstream trough that should reach the West by next
Tuesday-Wednesday. Meanwhile the combination of an initial
Rockies/Plains ridge and a faster moving southern Canada/northern
tier U.S. ridge will likely build into the East toward the middle
of next week. The mean evolution aloft along with a surface
pattern that should promote a multi-day feed of Gulf moisture will
favor the best potential for heavy rainfall over portions of the
Plains and Mississippi Valley. Weak shortwave energy filtering
through the generally rising heights aloft over the East may
extend some areas of meaningful rainfall into parts of the East.
Also expect some precipitation over portions of the Rockies while
the second Pacific trough should bring some rain and high
elevation snow into the Northwest. Double-digit temperature
anomalies should be mostly of the warm variety and are most likely
from the Northwest into the northern High Plains.


...Guidance Evaluation and Preferences...

Over the past day the GFS has made significant progress in
adjusting its Pacific through North America pattern faster toward
other guidance. So instead of developing a Great Lakes closed low
from northern tier/southern Canada shortwave energy, the GFS is
weaker and waits until New England and the Canadian Maritimes to
show amplification--a more plausible scenario with latest CMC runs
and GEFS/ECMWF means also suggesting this possibility. However
there are still issues with this energy and possibly other weak
low-predictability shortwaves over the East, creating a lot of
latitude spread for rainfall/convective systems that may extend
eastward from the Midwest. Preference is for an intermediate
solution, between the more amplified GFS/GEFS and least amplified
(with strongest ridging) 12Z ECMWF. Farther west, guidance is
still waffling a bit with specifics of the trough/upper low
initially over California but the consensus timing appears to be
fairly consistent through at least Monday followed by typical
divergence with some dependence on upstream flow. As for the
second western trough, a blend approach represents the mean
pattern well while downplaying the more extreme details such as
the 12Z ECMWF developing more southeastward amplitude than most
other guidance by next Wednesday. Late the period the new 00Z
ECMWF compares better to the guidance average in both respects.
Based on guidance comparisons the starting blend incorporated a
composite of operational guidance for the weekend and then trended
toward 40-70 percent total input of the 18Z GEFS/12Z ECMWF means
by days 6-7 Tuesday-Wednesday as confidence in various model
specifics declined.


...Weather Highlights/Hazards...

Early in the period expect weak shortwave energy and a wavy front
to focus one area of rainfall over parts of the Central Plains and
Midwest while improving Gulf inflow will begin to increase
rainfall over the Southern Plains. Some of the activity over the
Midwest could eventually reach into the Ohio Valley/eastern states
as the focusing front and shortwave energy extend farther
eastward, though currently there is some uncertainty over the
precise latitude of this axis. Confidence is somewhat greater for
a multi-day threat of heavy rainfall over the Southern Plains and
parts of the Mississippi Valley through early-mid week as the low
level flow of Gulf moisture should persist and the initial western
U.S. upper system emerges into the Plains. Parts of this area have
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 due to
potential for some easterly upslope flow along with the system
emerging from the West. In most cases any precipitation to the
west of the Rockies should be fairly light and scattered. Activity
reaching the Northwest late in the period may be somewhat better
organized but should still be on the lighter side of the spectrum.
Higher elevations may see some snow. The upper trough about to
depart from the East Coast may produce areas of mostly light
mid-late day rainfall from the northern Mid-Atlantic into the
Northeast during the weekend.

One or more days with highs of 10-15F or so above normal will
extend from the Northwest U.S. into the northern High Plains with
an upper ridge moving over the area and warm flow ahead of a
frontal system moving into the West. In contrast the system
crossing southern parts of the West should bring moderately below
normal highs to areas from the far southwestern U.S. into southern
High Plains. The Northwest should see highs drop to below normal
levels by next Tuesday-Wednesday behind the late-period front.
Meanwhile moderately cool temperatures over the eastern half of
the country during the weekend (though up to minus 10F or so
anomalies for morning lows over the southern
Mid-Atlantic/Southeast on Saturday) will moderate to yield
near-normal highs and somewhat above normal lows Monday-Wednesday.

Rausch/Santorelli