Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Sun Mar 26 2023
Valid 12Z Wed Mar 29 2023 - 12Z Sun Apr 02 2023
...Overview...
A low pressure system just offshore of California as the period
begins Wednesday morning could lead to some coastal rain and
mountain snow continuing across California into Wednesday and
Thursday. The upper trough and associated surface low is forecast
to push eastward through the latter part of the week and emerge
into the Plains. Thus precipitation will spread across the
Intermountain West Thursday and then into the central U.S.
Thursday-Friday. Rain and thunderstorms that could cause severe
weather and flash flooding are likely through much of the Plains
and Mississippi Valley, while wintry weather could affect the
north-central U.S. on the backside. As this system pushes east,
another upper trough is forecast to drop into the Northwest by
next weekend.
...Model Guidance/Predictability Assessment...
Recent model guidance is reasonably agreeable on the synoptic
scale through much of the medium range period. Trends have been
for a slightly weaker but still notable surface low near the
California coast for Wednesday, with good consensus on its
position. Meanwhile on Wednesday model trends have continued for a
well offshore and suppressed Atlantic low. As the West Coast upper
low tracks southeast into the West and east into the central U.S.
Thursday-Friday while ridging amplifies ahead of it stemming from
a Gulf of Mexico upper high, models show differences well within
typical spread for this timeframe. GFS runs and even the 06Z GEFS
mean indicated an embedded upper low within the trough by late
Friday into Saturday as it lifts into the east-central U.S.,
unlike the ECMWF/CMC that just show a potent shortwave. Again,
these are relatively minor differences but they are associated
with variations in the surface low placement and the amounts/axis
of precipitation that could be snow across the north-central U.S.
to the northwest of the surface low around Friday. Shortwaves
behind this main feature are more variable and could affect
periods of upper ridging behind it. By the weekend another larger
scale trough with possibly an embedded upper low is forecast to
come southeast into the Northwest. While there are some
differences in the timing and depth of this feature depending on
how the energy digs, true outliers were not really noted other
than perhaps the new 12Z CMC that was somewhat flat across the
Pacific Northwest as it splits energy differently. The WPC
forecast was thus based initially on a blend of the 00Z
deterministic guidance and then gradually increased the
proportions of ECMWF ensemble mean and less so the GEFS mean as
the period progressed to reduce the impacts of individual models
varying.
...Weather/Hazards Highlights...
As the period begins Wednesday, the surface and upper low just
offshore of California will be directing an (albeit somewhat weak)
atmospheric river into California. This plume of moisture is
likely to affect southern California on Wednesday before pushing
south into Mexico. Meanwhile the considerable upper divergence and
somewhat steep lapse rates caused by the upper low so nearby could
cause scattered thunderstorms across the central California coast
through Wednesday into Thursday. Precipitation amounts shown by
the models are fairly weak, but some ingredients for heavy
rainfall rates are in place so a Marginal Risk of excessive
rainfall was added for day 4/Wednesday. Meanwhile in the higher
terrain of the Sierra Nevada, additional snow is likely through
midweek, and snow should spread into the Intermountain West and
Rockies as well through Thursday. Then by late week into the
weekend, another east Pacific/western Canada trough should direct
moisture more into the Northwest this time, though still could
affect northern California, with lower elevation rain and mountain
snow possible in those areas.
As the first trough and low system from the West eject into the
Plains for the latter half of the week, precipitation will spread
into the central and then eastern U.S. before clearing out by next
Sunday. Strong to severe thunderstorms are possible in the
southern half of the Plains Thursday and into large portions of
the Mississippi Valley on Friday per the Storm Prediction Center's
extended outlook. The day 5/Thursday experimental Excessive
Rainfall Outlook also indicates a couple of Marginal Risk areas in
place. One is in the Southern Plains where moisture and
instability will be abundant and could cause high rain rates ahead
of a cold front. The instability axis will also push farther north
into the vicinity of a warm front centered in and around Iowa, so
flash flooding is possible there as well. Moderate to heavy rain
is forecast to shift into the eastern U.S. late week, though there
has been some model variety in amounts. Meanwhile on the backside
of the low, a round of snow is possible in the north-central U.S.,
though the positioning and amounts of notable snow totals will
have to be refined with time. Windy conditions also could be a
threat across the southwestern to south-central U.S. and
progressing eastward toward the Ohio Valley with time behind the
cold front.
Troughing in the West will lead to cooler than normal temperatures
there for much of next week, with highs around 10-20 degrees below
normal. The coolest anomalies are likely to be across the northern
Plains though, where temperatures will range from 10 to 30 degrees
below average depending on the placement and day, equating to
highs in the teens to 30s and lows near or even below 0F. After a
near average Wednesday in terms of temperatures elsewhere, ridging
extending from a Gulf of Mexico upper high should cause relative
warmth to rebuild across the southern/central Plains Thursday and
spread into the Mississippi Valley Friday and the East by
Saturday, with highs generally 10-15F above average and lows
around 15-25F warmer than normal. A few record warm low
temperatures could be tied or set, but record highs may be limited
to Florida.
Tate
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
