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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0700Z Apr 13, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sat Apr 13 2024

Valid 12Z Tue Apr 16 2024 - 12Z Sat Apr 20 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 entering the Central U.S. to start the period Tuesday
will support a deepening surface low pressure system which will
track into the Upper Midwest/Great Lakes Wednesday-Thursday.
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/low digging down from Western Canada should ultimately
evolve into a well organized upper low near/over the Upper Great
Lakes by later in the week with deeper troughing moving across the
Eastern third of the Nation next weekend. Behind this, upper
ridging looks to build over the Northwest/western Canada.


...Guidance/Predictability Assessment...

Model guidance remains reasonably agreeable on the large
scale/presence of these features, but still with plenty of
disagreement in the timing and details. With the initial low out
of the Plains early-mid week, there are some model run-to-run
timing variability still, but enough agreement to support a purely
deterministic model blend. The next system dropping out of Canada
into the Upper Great Lakes later next week shows much more
uncertainty on its evolution and placement, which translates to
differences in surface front progression across the Eastern U.S.
into next weekend. The models/ensembles generally agree that
ultimately a closed low forms over/near the Great Lakes, but the
12z/Apr 12 ECMWF was more the outlier solution showing an elongated
trough/upper low stretching from central Canada towards the Great
Lakes, which caused a much faster progression of the Great Lakes
upper low due to additional energy from Canada pushing it eastward
(and weaker ridging over the West). The GFS and CMC were more
compact, and this sort of solution had better agreement from the
ensembles and the ECMWF-initialized AI/ML models as well. And
interestingly enough, the new 00z ECMWF run tonight (available
after forecast generation time) came into better agreement with the
GFS/CMC and ensembles. Due to the increasing uncertainty in the
late periods, the WPC blend trended towards the ensemble means late
period, with some influence still from the GFS for better system
definition.


...Weather/Hazards Highlights...

Precipitation will be ongoing across the Rockies/Central U.S. with
multiple weather hazards as the upper low helps to deepen a Plains
surface low into Tuesday. The Storm Prediction Center has been
highlighting eastward progressing severe thunderstorm potential
from the Plains on Monday into the Lower to Mid- Mississippi
Valley 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 Upper
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
(Tues-Wed night) Excessive Rainfall Outlooks across portions of the
Northern Plains/Upper Midwest into the MS-Valley/Ohio
Valley/Appalachians. 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 Midwest on Tuesday, but for now there is
still too much spread to be confident in the placement. Behind the
low, the storm should produce a broad area of gusty winds focusing
mainly across the north-central High Plains on 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 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. Blowing snow is also a
concern that may cause impacts. Farther south, by around Thursday
or so, the surface front ahead of this trough may become parallel
to the upper low over the south-central to east-central U.S. The
combination of decelerating progression and some Gulf inflow may
lead to increasing rainfall near the front around that time.

Expect a broad area of above normal temperatures over the eastern
half of the country into the southern Plains Tuesday-Thursday,
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 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, followed by
readings 10-15F below average over the northern Rockies/High
Plains by next Wednesday-Friday. Less extreme cool anomalies may
extend farther southwestward underneath amplified troughing late
week-next weekend across much of the eastern half of the nation.


Santorelli


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