Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
311 PM 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 upper low entering the central High Plains to start the
period Tuesday will support a deepening surface low pressure system
that tracks 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 into the Northwest Tuesday should ultimately
evolve into a well-organized upper low near/over the Upper Great
Lakes and amplify troughing in the eastern third of the nation by
late week. 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 is some model run-to-run timing
variability still, but enough agreement to support a purely
deterministic model blend. The next system tracking from the
Northwest into the Upper Great Lakes shows more uncertainty on its
evolution and placement, though with some model convergence in
recent cycles. Around midweek, most models are more agreeable than
a day ago regarding how much troughing will get into the northern
Rockies/High Plains, affecting snow totals there. The 00Z CMC was
the main outlier in showing less troughing, but the 12Z CMC was in
better agreement. There were also some differences Wednesday in
when this energy consolidates into a closed upper low, with GFS
runs being slower to close it off, but the 12Z GFS has come into
better alignment with that timing.

Models then show this upper low tracking eastward through south-
central to southeastern Canada Thursday-Saturday, though with
variations on the exact timing as well as the southern extent of
the trough amplitude into the U.S. At least recent model cycles do
not show too many axis differences with the trough/low compared to
the 12Z 4/12 run of the ECMWF for example. The details of Northwest
ridging and Southwest southern stream troughing could also use
more time to determine. Thus the WPC forecast gradually
transitioned from a multi-model deterministic blend favoring the
GFS and ECMWF early in the period to a half model/half ensemble
mean blend by Day 7.


...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
reaching 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. Excessive Rainfall Outlooks
continue to show Marginal Risks stretching from the Mid-Lower
Mississippi Valley Tuesday into the Tennessee and Lower Ohio
Valleys Wednesday along with the potentially severe storms, while
farther north along the low track the ample lift should provide
support for possibly heavy rainfall. Watching a corridor from
eastern Nebraska and South Dakota into parts of Iowa and southern
Minnesota for an embedded Slight Risk on Tuesday, where heavier
totals are likely to maximize. This area will likely see exceedance
of Flash Flood Guidance values there, but in this region of the
country this does not often result in notable flooding impacts, so
held off. Additionally, this area has seen normal to below normal
soil moisture/streamflow conditions, another mark against
sensitivity to flooding. 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 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
notable snow will be likely over the higher elevations. Chances for
meaningful snow in lower elevations are decreasing, but blowing
snow is a concern. Then as systems progress, rain and convection
are forecast to move into the eastern U.S. into late week, while
lingering across the south-central U.S. as the frontal system
stalls.

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 upper trough digging into the West and then
including the northern Plains will likely start to bring below
normal highs into the Northwest by Tuesday, followed by readings
10-15F below average over the northern Rockies/High Plains by next
Wednesday-Thursday while gradually expanding farther south and east
behind the secondary cold front late week.


Tate/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