Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
429 PM EDT Wed Apr 20 2022
Valid 12Z Sat Apr 23 2022 - 12Z Wed Apr 27 2022
...Significant late season winter storm increasingly likely for
the Northern Plains this weekend and potentially heavy rainfall
threat to some areas farther south...
...Weather Pattern Overview...
A highly amplified pattern will prevail over the weekend into the
start of next week as a deep upper low tracks northeastward
through the Northern Plains and a strong Southeast ridge extends
an axis northward. A strong surface low forecast to be over the
Dakotas on Saturday and continue northeast thereafter, along with
the trailing front, will bring a wide variety of weather hazards -
significant snow and wind to the Northern Plains and eventually a
heavy rain threat evolving late in the weekend into early next
week over portions of the Southern Plains to the Ozarks and Mid
Mississippi Valley. By Tuesday-Wednesday, gradual progression of
the mean pattern will bring moderate upper troughing to the East,
a ridge to the Rockies/Plains, and a trough into the eastern
Pacific/West Coast. Much below normal temperatures may persist for
a while over the far northern parts of the Plains while well above
normal readings are possible over a majority of the eastern half
of the country through the weekend.
...Model Guidance Evaluation...
The updated forecast emphasized a blend of the 00Z ECMWF/00Z
UKMET/06Z GFS during the first half of the period, followed by
gradually increasing input of the 06Z GEFS/00Z ECMWF means so that
the means had half total weight by day 7 Wednesday. Recent ECMWF
runs have generally provided the best intermediate timing for the
northern tier storm, with GFS runs consistently faster and the CMC
slow. UKMET runs have tended to be between the ECMWF and CMC but
the new 12Z UKMET has adjusted to the ECMWF, leaving the CMC more
in the slow minority. Meanwhile guidance has varied considerably
over recent days for the amplitude and timing of the trailing
upper trough that progresses through the central/eastern U.S.,
affecting the timing and waviness of the cold front trailing from
the surface low. Over the past day most guidance has been
consolidating toward greater progression than recent CMC runs. It
is possible that low pressure could evolve near New England by day
7 Wednesday depending on specifics of the eastern North America
upper trough. For the eastern Pacific/West Coast upper trough,
from late Sunday onward there is still a lot of model/ensemble
spread with respect to individual shortwaves flowing around the
trough and the eventual location of the embedded upper low. By
next Wednesday the new 12Z ECMWF/CMC brings the upper low closest
to the Pacific Northwest while the 00Z ECMWF mean kept it well to
the northwest. Prefer a compromise approach within this broad
spread until better agreement emerges.
...Sensible Weather/Threats...
A strong to potentially significant storm system will likely track
over the Northern Plains and Upper Midwest during the weekend. The
best potential for significant totals of snow and blowing snow
extends from northeast Wyoming and eastern Montana through much of
western/central North Dakota and far northwest South Dakota -
immediately west to northwest of the surface low track. The strong
winds and heavy snow combined could lead to near blizzard
conditions for some areas. There is some lingering uncertainty in
the speed and track but the gradually improving guidance cluster
suggests good confidence in a high impact late season winter storm
for portions of the region where the latest Winter Weather Outlook
shows high probabilities for at least several inches of snow.
The frontal boundary anchored by this storm will help to focus an
area of heavy rainfall from parts of the southern/central Plains
toward the Ozarks and Mid Mississippi Valley. Southerly flow of
warm, moist air intersecting the boundary will likely lead to
strong thunderstorms that may train or repeat over the same areas
Sunday into Monday. Mesoscale convective systems repeating over
the area could lead to an axis of multi-inch totals and the latest
guidance suggests the best potential for flash flooding and
excessive rainfall from far northeastern Texas through eastern
Oklahoma, northwestern Arkansas, and southern Missouri along with
adjacent areas. The Day 5 Experimental Excessive Rainfall Outlook
shows a Slight Risk over this region given the potential, as
stream flows and precipitation anomalies are already above normal
due to recent heavy rainfall and with the potential for additional
heavy rainfall leading up to Day 5. Guidance is still in the
process of resolving the timing and wave details of the surface
front so changes in the most likely coverage and magnitude of
rainfall totals are certainly possible. Lighter rainfall will
spread through the eastern U.S. during Tuesday-Wednesday as the
front accelerates.
Mostly light precipitation over the northern part of the Pacific
Northwest to start the week should expand southward through Oregon
and eastward into the northern Rockies after Monday as an upper
trough approaches the West Coast. Some pockets of moderate
activity are possible. Day-to-day details are still uncertain due
to lower confidence in specifics of shortwaves within the overall
upper trough.
Expect unseasonably cold temperatures to persist over far northern
parts of the Plains through the period with North Dakota in
particular seeing highs 15-30F below normal for multiple days.
While still below normal, anomalies may finally moderate some
around midweek. The West will see more variable temperatures, with
minus 5-15F anomalies over the Intermountain West/Rockies during
the weekend followed by a west-to-east warming trend with
increasing coverage of above normal readings. Warmest highs of
10-15F above normal are likely to extend from the Great
Basin/Southwest into the central High Plains during
Tuesday-Wednesday. At the same time clouds/precipitation ahead of
the upper trough nearing the West Coast may bring slightly below
normal highs to the Pacific Northwest. Much of the eastern half of
the country will see a significant warming trend during the
weekend and continuing into early next week over the East. Plus
10-20F anomalies should be fairly common for one or more days and
parts of the Midwest/Great Lakes could see morning lows 20-25F
above normal. One exception to the eastern warmth may be over the
Northeast/northern Mid-Atlantic where cool surface high
pressure/ridging may prevail for a while. A front meandering over
the Mid-Atlantic could provide a sharp contrast between cool air
to the north and warmth to the south and west, increasing the
challenge of temperature forecasting over the region.
Rausch/Taylor
Hazards:
- Heavy precipitation across portions of the far northern Plains,
Sat, Apr 23.
- Heavy rain from across portions of the central and southern
Plains into the Midwest, Sat-Sun, Apr 23-Apr 24.
- Heavy snow across portions of eastern Montana and western North
Dakota, Sat, Apr 23.
- Severe weather across portions of the central Plains west of the
middle to upper Mississippi Valley, Sat, Apr 23.
- Flooding possible across portions of Georgia and the northern
Plains.
- Flooding occurring or imminent across portions of the Southeast
and central Mississippi.
- Flooding likely across portions of the Southeast.
- High winds across portions of the northern Plains, Sat-Sun, Apr
23-Apr 24.
- Much below normal temperatures across portions of the northern
Plains, Sat-Tue, Apr 23-Apr 26.
- Enhanced wildfire risk from the foothills of the southern
Rockies to portions of the central Plains, Sat, Apr 23.
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, experimental excessive rainfall
outlook, winter weather outlook probabilities and heat indices are
at:
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