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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1915Z May 18, 2024)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
314 PM EDT Sat May 18 2024

Valid 12Z Tue May 21 2024 - 12Z Sat May 25 2024


Multiple progressive upper shortwaves/possible closed lows and
surface low pressure/frontal systems will traverse the West to
northern/central Plains and Midwest during the medium range period.
For Tuesday- Wednesday, a potent shortwave will send a modestly
deep surface low into the Upper Midwest and potential for at least
locally heavy rain and thunderstorms across the region. Its
attendant cold front will push across the East and partway into the
South midweek and beyond, with rain and storms along and ahead of
it and cooler temperatures behind. Heat will continue to be
hazardous for at least southern Texas though. Another couple rounds
of energy may push through the Northwest eastward late week, but
with much more uncertainty.

...Guidance/Predictability Assessment...

Models show reasonable agreement with the first shortwave that
kicks out into the Upper Midwest Tuesday-Wednesday, though with
some uncertainty in the timing and depth of the surface low with
the potential merging of upper-level energies. The next upper low
tracking through the Northwest Wednesday and eastward late week
starts with slightly timing better agreement compared to runs from
a day ago, as 06Z and new 12Z GFS runs are more agreeable in
showing this feature separate from upstream energy, similar to the
non- NCEP guidance. There is still some spread in the deterministic
and AI models though. By Saturday, the 00Z deterministic ECMWF
seemed to become an outlier in producing ridging over the north-
central U.S. as it is faster to shear out the shortwave energy.
Other models including EC-based AI models prefer troughing if not
the upper low remaining closed there, and the 12Z ECMWF trended
toward this, though north with its center into south-central
Canada. Then upstream, models generally show yet another closed low
dropping south into the Northwest for the end of the week. The 00Z
ECMWF was west of other operational and AI models with its
position by Saturday. Another feature of possible concern but on
the smaller scale is a southern stream shortwave moving through the
southern Plains to Mid-South to Mid-Atlantic Thursday-Saturday,
which has implications for QPF among other sensible weather, so
this will continue to be monitored.

Thus the WPC forecast utilized a blend of the deterministic
guidance early in the period, but introduced and increased the
proportion of ensemble means to over half by the end of the period
given the model spread. This maintained reasonable continuity with
the previous forecast, though slowed the central U.S. surface low
and front by Saturday with most models trending that way.

...Weather/Hazards Highlights...

An upper shortwave and leading cold front, with deepening surface
low, will push into the Upper Midwest Tuesday/Wednesday, helping to
fuel showers and thunderstorms across this region. Ample moisture
will be in place while the low track and upper level flow provide
significant lift for heavy rain on Tuesday. Thus will embed a
Slight Risk in the Day 4/Tuesday Excessive Rainfall Outlook across
parts of the Upper Midwest. Notable instability will be present on
the warm side of the low, which will also provide support for
likely severe thunderstorms over the Middle Mississippi Valley
according to the Storm Prediction Center. But, a limiting factor
for flash flooding farther north will be the lack of instability on
the backside of the low, despite model guidance showing heavy
amounts across northern Minnesota for example. A broader Marginal
Risk extends around and south just ahead of the cold front. After
Tuesday, expect northeastward progression of the surface low to
push the trailing cold front and accompanying rain/storms farther
east and south with some more potential for heavy rainfall in the
eastern and southern U.S. for Wednesday-Thursday. The front is
likely to slow or stall west-east in a moist and unstable
environment, and with multiple rounds of convection with possible
heavy rain rates, will embed a Slight Risk in the ERO for Day
5/Wednesday from around Texarkana into the Mid-South. Also expanded
the Marginal Risk into west- central Texas with possible dryline
convection with heavy rain rates. Approach/arrival of another
couple of systems as currently advertised into the Northwest by
midweek and next weekend would produce somewhat more organized
precipitation there, and potential for heavy snow in the mountains,
with snow levels dependent on the depth of the upper low(s).

Expect South Texas to see multiple days of hazardous heat during
the period with highs persistently running 10-15F above normal with
max heat index values possibly reaching at least 110F. Highs near
100 degrees could stretch farther north across the southern High
Plains at times as well. Some daily records for highs/warm lows
will be possible. Above normal highs will also track east across
the Midwest and Northeast into Wednesday but should moderate by
Thursday-Friday as the cold front pushes through the region. The
forecast pattern will favor below average highs over the Northwest
to northern Plains for most of next week.


Additional 3-7 Day Hazard information can be found on the WPC
range hazards outlook chart at:

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from: