Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
137 AM EDT Sun Mar 21 2021
Valid 12Z Wed Mar 24 2021 - 12Z Sun Mar 28 2021
...Midweek heavy rain threat over parts of the South, most likely
from the central Gulf Coast into southern Appalachians...
...Overview...
Expect a large scale mean trough aloft to persist over the
Plains/west-central U.S., between a ridge over the eastern Pacific
and a strengthening Cuba/Bahamas ridge whose influence will extend
into parts of the Atlantic and Gulf of Mexico. Shortwaves within
the trough will spread areas of rain and mountain snow from the
Pacific Northwest through much of the Rockies, and with the help
of Gulf moisture generate a broad area of rainfall over the
eastern half of the country. The greatest potential for heavy rain
currently exists from the central Gulf Coast into the southern
Appalachians. Models and ensembles still agree on the overall
pattern while gradually attempting to come close together or
stabilize for important timing and detail aspects of the most
significant features.
...Model Guidance/Predictability Assessment...
One shortwave and associated deep surface low will be exiting the
Upper Midwest Wednesday-Thursday, as a second follows on its heels
(and a bit farther south) from the Southern Plains to the
Northeast Thursday-Saturday. The deterministic models show fairly
good agreement at this time with both systems so WPC leaned on a
majority deterministic model blend for days 3-5. Bigger forecast
questions and uncertainty begin to arise by late week as shortwave
energy amplifies over the Western U.S. and shifts east into the
Plains by Sunday/Day 7. The latest (18z/Mar 20) run of the GFS
splits northern and southern stream energy more than the rest of
the guidance, developing a deep closed low over Southern
California by the end of the period. There is a little support for
this from the latest run of the CMC as well. However, run to run
continuity is poor for both pieces of guidance (as well as the
ECMWF which is quicker to bring the trough into the Plains) so a
blend towards the ensemble means was preferred for days 6 and 7.
...Weather/Threats Highlights...
Some lingering precipitation (rain and/or snow) is likely on the
north side of the surface low exiting the Upper Midwest on
Wednesday. South of this however, the interaction of Gulf inflow
with the trailing front--which should decelerate and possibly
stall for a time while awaiting ejection of upstream dynamics from
the West--is likely to produce heavy rainfall over parts of the
South around midweek. Locations from the central Gulf Coast region
into the southern Appalachians continue to show the best potential
to see the highest rainfall totals with this event. There is still
enough uncertainty that nearby areas will need to monitor
forecasts as well. Expect the aforementioned western energy to
generate a wave tracking west of the Appalachians Thursday into
Friday. This system will spread areas of moderate to heavy rain
through more of the East late in the week, with somewhat lower
totals than over the South due to faster progression/shorter
duration. Another area of rain may develop over the South next
weekend as a warm front returns northward ahead of the next upper
trough reaching the West. The two systems affecting the West will
bring rain and mountain snow from the Pacific Northwest
eastward-southeastward into much of the Rockies, with highest
totals over favored terrain in the Olympics, northern Cascades,
and northern-central Rockies.
The large scale pattern from Tuesday into the weekend will tend to
favor above normal temperatures over the eastern half of the lower
48 and below normal readings from the West into the Rockies and
parts of the High Plains. Expect the highest warm anomalies of
plus 10-25F to extend from the Great Lakes into the Northeast
Wednesday-Friday, ahead of the system tracking northeast from the
Plains and trailing wave lifting through the east-central U.S..
Morning lows could be close to daily record warm values over parts
of the East Coast. Proximity of the strengthening Cuba/Bahamas
upper ridge may push Florida Peninsula high temperatures close to
daily records at sites with a relatively shorter period of record.
Farther west there will be two episodes of coolest temperatures.
One around Wednesday should feature highs 10-20F below normal over
the Great Basin and central/southern Rockies. The second will
likely progress from west to east Thursday-Saturday with most
highs 5-10F or so below normal, possibly 10-15F below normal over
the north-central Rockies/High Plains by next Saturday.
Santorelli/Rausch
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, 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml