Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
402 PM EDT Thu May 12 2022
Valid 12Z Sun May 15 2022 - 12Z Thu May 19 2022
...Heat threat for the Desert Southwest early next week while
persistent southern Plains heat intensifies and expands eastward
into the Southeast...
...Overview...
A trough-ridge-trough pattern aloft across the U.S. northern tier
is forecast to make gradual progress eastward next week, such that
the mean troughs should reach near the East and West Coasts around
Wednesday. Ridging early in the week across the southwestern to
south-central U.S. followed by rising heights over the Southeast
should lead to a broad area of unseasonably hot weather across the
southern U.S., with the most persistent record-breaking heat
centered in Texas and Louisiana. Meanwhile showers and
thunderstorms of varying intensity are possible along fronts
pushed by the mean trough crossing the eastern U.S. and with
stalling fronts across the Plains. The West should see cooling
temperatures in response to upper troughing as the week
progresses, along with lowering snow levels by midweek within
rounds of precipitation.
...Guidance Evaluation/Preferences...
Recent guidance continues to be agreeable with the mean pattern,
but with lingering differences with low-predictability shortwave
details within the eastern U.S. trough as well as for shortwaves
moving through and ejecting from the Pacific trough eventually
pushing into the West. Day-to-day frontal position and thus
specifics of convective focus will be sensitive to these uncertain
medium to smaller scale aspects of the forecast that may take
until the short range to resolve. Over the past few days, CMC runs
have been generally slow and deep with the eastern trough around
Mon-Tue. This persisted in the 00Z run but to a lesser extent,
while the newer 12Z run appears even closer to consensus
fortunately. CMC runs have also been more amplified than most
other guidance with the Pacific to western U.S. troughing later
Tuesday into Wednesday-Thursday--though this feature begins
showing more spread in general by midweek onward among
deterministic models and GEFS/EC/CMC ensemble members as not only
uncertain shortwaves pivot, but as stronger energy stemming from
an initial upper low spinning just west of British Columbia could
start spilling southward. Models vary with whether energy will
consolidate into one upper low/trough feature or possibly two
upper lows/troughs, with the latter shown by the 00Z CMC and 12Z
GFS for example. For the updated WPC forecast, began with a blend
of the 00Z/06Z deterministic guidance led by the ECMWF and GFS
runs, then phased out the CMC (and the UKMET as it ends) in favor
of gradually increasing proportions of the GEFS and EC ensemble
means that were reasonably agreeable to downplay the more
uncertain details.
...Weather/Threat Highlights...
Initial ridging across the Southwest and southern Plains and then
eventually rising heights farther east over the Deep South will
likely promote a broad area of heat across much of the southern
U.S. during the period. The greatest and most lasting anomalies
through the period should be over Texas (persisting from the short
range time frame) where some locations could see highs up to
15-25F above normal. The southern half or so of the West will tend
to see highs up to 10-15F above normal for the early part of the
week, followed by a slight cooling trend that reaches California
initially and extends farther eastward by next Thursday as Pacific
upper troughing arrives. Meanwhile by Tuesday or Wednesday through
Thursday the coverage of highs 10-15F above normal should extend
eastward through the Lower Mississippi Valley and into the
Southeast. Potential for daily record highs will be most
persistent through the period over parts of the southern Plains
into Louisiana, with numerous records likely to be set.
In contrast, the series of surface waves/fronts ahead of the
approaching/arriving eastern Pacific upper trough will keep the
Pacific Northwest on the cool side with frequent episodes of
precipitation. The trough should reach far enough eastward to
bring highs down to 10-15F below normal over the
Northwest/northern Rockies by next Wednesday-Thursday. Snow levels
should be quite high into early next week and then begin to
decline as the upper trough arrives. It will likely take into the
short range time frame to resolve timing/intensity aspects of each
precipitation episode due to the medium to smaller scale of the
supporting shortwaves.
Areas to the east of the Rockies should generally see two rainfall
regimes during the period. From Sunday into Monday the best focus
for showers and thunderstorms will be along a cold front pushing
through the East. There may be one area of emphasis over and near
the south-central Mississippi Valley on Sunday while additional
areas of locally moderate to heavy rainfall may occur from the
Great Lakes/Ohio Valley into New England around Monday. There
could be some localized flooding impacts over New England given a
suitable environment for high rainfall rates, but dry soils and
the front being progressive may temper effects. Then from later
Monday through Thursday, the stalling of the leading front and
another one behind it over the Plains along with shortwaves aloft
will provide focus for multiple areas of showers and
thunderstorms/convective complexes across the northern half of the
Plains and Mississippi Valley into the Ohio Valley and central
Appalachians. Some rainfall may be heavy, with some added time
required to refine the details of coverage and timing of highest
totals.
Tate/Rausch
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
Hazards:
- Heavy rain across portions of the Southern Appalachians, the
Central Appalachians, and the Ohio
Valley, Wed-Thu, May 18-May 19.
- Flooding possible across portions of the Upper Mississippi
Valley and the Northern Plains.
- Flooding occurring or imminent across portions of the Lower
Mississippi Valley, the Upper
Mississippi Valley, and the Northern Plains.
- Flooding likely across portions of the Upper Mississippi Valley
and the Northern Plains.
- Excessive heat across portions of the Central Great Basin,
California, and the Southwest,
Sun-Tue, May 15-May 17.
- Excessive heat across portions of the Lower Mississippi Valley
and the Southern Plains, Sun-Thu,
May 15-May 19.
- Excessive heat across portions of the Southeast, the Lower
Mississippi Valley, the Southern
Appalachians, and the Tennessee Valley, Tue-Thu, May 17-May 19.
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, experimental excessive rainfall
outlook, winter weather outlook probabilities and heat indices are
at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml