Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
301 PM EDT Mon Apr 19 2021

Valid 00Z Tue Apr 20 2021 - 00Z Thu Apr 22 2021

...Snow continues to spread south into the Central Rockies/High Plains
this evening before swinging out into the Central Plains, Midwest, and the
Lower Great Lakes on Tuesday...

...Record cold and subfreezing temperatures expected across much of the
south-central United States on Wednesday morning...

...Additional rounds of heavy rain to continue across central Florida over
the next few days...

A late-season upslope snow event is in progress from Wyoming to northern
Colorado as a surge of cold air from Canada continues to plunge southward.
The cold surge will bring rapidly falling temperatures as well as a quick
burst of snow through the northern and central Rockies/High Plains tonight
before overspreading much of Kansas and southern Nebraska by early Tuesday
morning. As a low pressure wave forms along the sharp cold front, the snow
will then swing eastward through the Midwest and the Lower Great Lakes on
Tuesday. The highest additional snowfall amounts are expected to occur
along the north-central Colorado Rockies, where an additional foot of snow
could fall. Meanwhile, over 2 inches of snow is likely across the northern
half of Kansas and northwest Missouri. Winter Storm Warnings and Winter
Weather Advisories remain in effect for these regions. As the area of low
pressure moves towards the Great Lakes on Tuesday, a few inches of wet
snow can be expected to accumulate between central Illinois to northwest
Ohio. Snow will struggle to accumulate during the daytime hours given the
high April sun-angle, therefore much of the snow will stick to grassy
surfaces and while falling at night. By Wednesday, much of the wintry
precipitation and accumulating wet snow will be found across northern and
western portions of New York State and the northern Appalachians.

Unseasonably cold air behind this strong cold front will lead to
widespread freeze and frost concerns beginning tonight across Midwest and
central/southern Plains. Numerous daily record low temperatures are
forecast to be tied or broken on Wednesday morning as temperatures dip to
around 20 degrees below average. Hard Freeze Warnings and Watches have
been issued in eleven states from North Texas to western Ohio.

Meanwhile, active weather is expected to linger across central Florida as
low pressure waves develop along a slow-moving front. Occasional showers,
heavy at times, together with embedded thunderstorms can be expected to
continue through Tuesday. A few storms could turn severe. In addition,
flash flooding will be a concern due to upwards of 5 inches of rain
forecast to fall across central Florida during the next few days. By
Wednesday morning, the rain chances should slowly diminish and edge
southward into southern Florida.

Snell
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
417 PM EDT Mon Apr 19 2021

Valid 12Z Thu Apr 22 2021 - 12Z Mon Apr 26 2021

...Record cold likely for parts of the central U.S. Thursday
morning...
...Heavy Rainfall/Flooding Threat across the South into the
weekend...

...Overview...

Blocky upper ridging forecast over Alaska and Greenland at high
latitudes and over the Greater Antilles in lower latitudes favors
upper troughing over much of the CONUS. As an upper low south of
Alaska splits off its southern portion toward the West Coast this
weekend, additional troughing will sink southward out of Canada
through the Great Lakes. Ridging will build in between these two
systems next week across the Plains. The pattern is rather chilly
for late April with precipitation chances focused over the South,
East Coast, and the West.

...Guidance/Predictability Assessment and Preferences...

Model solutions seem better clustered today with the mid-larger
scale flow evolution days 3-7 despite a pattern over the lower 48
states with multiple streams and the surrounding blocky flow.
System timing differences remain evident, but seem mitigated by a
favored composite blend of the 06 UTC GFS, 00 UTC
ECMWF/UKMET/Canadian and the 13 UTC National Blend of Models. This
blend is consistent with ensembles/newer 12 UTC models and
maintains good WPC product continuity.

...Weather/Hazards Highlights...

A southern stream system ejecting from the Southwest and southern
Rockies Fri will work over the Southern Plains where rainfall will
expand along and ahead of a warm front with return of Gulf of
Mexico moisture and instability. Heavy rainfall is likely from the
southern Plains through the lower MS Valley and Southeast Fri-Sat.
Runoff and flooding issues are expected, especially given portions
of the region have received much above normal rainfall in April.
The system may then lift up the East Coast under influence of
northern stream energy from the Great Lakes to bring enhanced
rainfall across the Mid-Atlantic and windy New England Sun/Mon.

Meanwhile well upstream, an amplifying Pacific upper trough/low
and surface front will work into the West Coast over the weekend
into early next week, eventually bringing a lower elevation
rain/mountain snow focus into CA, mainly the Sierra.

Areas east of the Rockies will see below normal temperatures
Thursday, perhaps by 10-20 degrees which would support record cold
low temperatures from Texas northeastward toward the Corn
Belt/Midwest. That cold air mass will push eastward but moderate
into the Northeast/Mid-Atlantic. With northern stream troughing
nosing southward out of Canada, areas along the border will see
below normal temperatures as well. This should support moderately
heavy snow over the north-central Rockies Thu into Fri with
upslope. An area of above normal temperatures will be over parts
of the West/Great Basin ahead of the upper trough/low while
offshore. As that trough works inland into early next week, the
milder air should shift eastward across the Rockies and into the
Plains.

Fracasso/Schichtel