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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0657Z Apr 12, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Fri Apr 12 2024

Valid 12Z Mon Apr 15 2024 - 12Z Fri Apr 19 2024

...Deepening Plains surface low likely to bring severe and heavy
rainfall threats ahead of it and strong winds and notable snow
behind...


...Overview...

A closed low over Nevada/California to start the period Monday will
progress eastward eventually supporting a deepening low pressure
system over the Plains by Tuesday. Various hazards are likely with
this storm system, including severe weather and bands of heavy
rainfall on the warm side along with a broad area of gusty winds.
Behind this storm, another upper trough digging down from British
Columbia into the Northwest U.S. early next week should ultimately
evolve into a broader positively tilted mean trough from south-
central Canada through the West. Expect this trough to bring colder
temperatures and multiple days of snow potential especially to the
northern half of the Rockies/High Plains. Meanwhile upper ridging
over the Plains on Sunday will shift eastward and settle near the
East Coast Tuesday-Thursday, leading to a corresponding shift for
above normal temperatures.


...Guidance/Predictability Assessment...

Models continue to show good agreement on the upper low over CA/NV
as it slides eastward inducing strong cyclogenesis in the Central
U.S. early next week. Including the new 00z runs tonight, there
remains some uncertainty in the exact placement and timing of the
surface low and much more uncertain low development off the
Northeast Coast next Thursday as the upper low weakens and a
shortwave moves through New England later in the week.

Behind this, another sharp trough/possible closed low will skirt
the Northwest/Northern Plains mid-week, eventually shearing with
energy continuing eastward through the Upper Great Lakes, and
energy hanging back over the Northwest next Thursday-Friday. Lots
of uncertainty in the evolution of this though.

The blend for tonights WPC forecast used an equal blend of the
deterministic models days 3-5 (minus the CMC which was slower than
the other guidance with the Central U.S. surface low). After this,
increased the ensemble means to 60 percent of the blend by Day 7,
along with the ECMWF and GFS to maintain some system definition.
Overall, this maintained good continuity with the previous WPC
shift as well, through Thursday.


...Weather/Hazards Highlights...

Precipitation will be increasing in coverage across the
Rockies/Central U.S. with multiple weather hazards as the upper
low helps to deepen a Plains surface low Mon-Tues. The Storm
Prediction Center is already highlighting areas of severe potential
for the Plains on Monday and for the Mid-Mississippi to western
Ohio Valley on Tuesday given the abundant instability and moisture
present with this system. These ingredients could also produce
heavy rainfall with intense rates capable of flash flooding
over parts of the Plains and the Midwest. Guidance continues to
show a fair amount of scatter with the details, and some areas
have dry antecedent conditions that could limit the potential, but
the overall threat remains sufficient to show very broad Marginal
Risk areas on both the Days 4 and 5 (Monday-Tuesday night)
Excessive Rainfall Outlooks progressing from the northern/central
Plains into the Upper Midwest/Mississippi Valley. An embedded
Slight Risk could be added if and when models narrow in on a focus
for heavy rainfall, especially across parts of the Upper MS Valley
on Tuesday where models are showing some agreement for higher QPF
(which should be fairly progressive). Behind the low, the storm
should produce a broad area of gusty winds, especially over the
southern/central High Plains Monday and into the north-central High
Plains Tuesday.

The next upper trough digging into the western U.S. and eventually
extending into the northern Plains early-mid week, along with
another frontal boundary, should tend to focus precipitation over
the northern half or so of the Rockies and High Plains for a few
days. Thus on the backside of the central U.S. surface low,
meaningful snow will be possible over at least the higher
elevations, and some snow may extend into the High Plains by
midweek as colder air reaches the region. By next Thursday or so,
the surface front ahead of this trough may become parallel to the
upper low over the central U.S. The combination of decelerating
progression and some Gulf inflow may lead to increasing rainfall
near the front around that time.

Plains/Mississippi Valley upper ridging will support well above
normal temperatures across the central U.S. into Monday with some
northern/central areas seeing highs 20-25F above normal.
Thereafter, the pattern evolution will lead to a broad area of
above normal temperatures over the eastern half of the country into
the southern Plains, with advancing frontal systems slowly
trimming the western side of the warmth. Expect highs in the
eastern U.S. to be up to 10-15F or so above normal with anomalies
for morning lows tending to be several degrees higher. The
southwestern U.S. will see below normal highs through Monday with
the upper low crossing the region. Then the upper trough digging
into the West and then including the northern Plains will likely
start to bring below normal highs into the Northwest early next
week and followed by readings 10-20F below average over the
northern Rockies/High Plains by next Wednesday-Friday. Less
extreme cool anomalies may extend farther southwestward.


Santorelli/Tate


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

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:

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
https://www.wpc.ncep.noaa.gov/#page=ovw