Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 PM EDT Fri Mar 17 2023
Valid 12Z Mon Mar 20 2023 - 12Z Fri Mar 24 2023
...Another strong storm with heavy rain/snow and wind threats to
impact California...
...Overview and Guidance/Predictability Assessment...
Latest guidance agrees fairly well upon the idea of the large
scale upper pattern featuring a mean trough along or a little
inland from the West Coast. The most prominent system within this
pattern should reach the West early-mid week with an atmospheric
river containing some moisture of tropical origin, and then track
across the Plains/east-central U.S. later in the week with an
expanding area of precipitation over that part of the country.
Another system dropping into the mean trough may reach the West
Coast by next Friday.
The main focus forecast-wise during the period will be on the
transition into a more energetic and amplified pattern over the
CONUS as at least a couple upper-level systems enter the picture.
At the start of the period, an upper-level low over the Northeast
will be departing over the Atlantic with energy lingering to the
southwest as a persistent upper-level low remains in place far
upstream over the Gulf of Alaska. A weak southern stream shortwave
will be crossing the Southern Plains and moving into the
Southeast, with the 00Z CMC still a bit ahead of the other
guidance, similar to previous runs. The prior few runs of the GFS
continued to show this wave deepening near the Atlantic Coast, and
this is even hinted at in recent runs of the ECMWF. However, the
00Z and particularly the 06Z GFS runs have once again backed off
this trend. The guidance has also come into better agreement with
an initial batch of energy ejecting from the low over the Gulf of
Alaska as a compact shortwave over the Pacific Northwest early in
the forecast period. Attention then turns to the Gulf of Alaska
low dropping southward and moving over the West by mid-period,
helping to initiate a stream of moisture over the Pacific and lead
to another Atmospheric River event for California. The guidance is
in fairly good agreement on the evolution of this pattern on a
larger-scale, but potentially significant disagreements pop up in
the handling of embedded energy, specifically a shortwave/compact
upper low translating northward over the central U.S. that has
implications for surface cyclo/frontogenesis over the Plains. The
00Z CMC is a bit more aggressive overall in depicting a much
stronger deepening low as opposed to a weaker low/open wave in
recent runs of the ECMWF and GFS. The GFS and ECMWF also show a
bit more amplified ridge downstream ahead of the troughing over
the West. Details become even more cumbersome by the end of the
period as another trough begins to dig in the West. This general
pattern is captured in the latest 00Z/06Z GFS and 00Z ECMWF runs
as well as the 00Z GEFS and ECens means, while the 00Z CMC is less
aggressive and only hints at a weaker shortwave over the
Southwest. More specific details in the evolution of potential
split-stream impulses in the broader mean troughing downstream
over the East is not consistent either run-to-run within the
individual guidance or in comparison between different models.
Even the 00Z GEFS/ECens means show potentially significant
differences with the GEFS mean more amplified with both of the
more significant trough features late in the period.
The WPC forecast blend begins with contributions from the 06Z GFS
and 00Z ECMWF/UKMET and a smaller influence from the 00Z CMC given
differences with respect to the southern stream shortwave.
Contributions from the 00Z GEFS/ECens means are added as the
influence of the 00Z CMC is removed in the middle of the period as
this solution diverges more significantly in the handling of the
evolution of the pattern over the central U.S. mid-late period
compared to the other guidance. The 00Z GEFS/ECens means are
relied on more heavily towards the end of the period given the
rapidly increasing uncertainty with respect to smaller-scale
shortwaves in the flow. The means help to capture the broader
pattern between the initial trough moving from the central to
eastern U.S. and the next trough digging in the West.
...Weather/Hazards Highlights...
A fairly weak leading system will support areas of rain and higher
elevation snow that should be in progress across a decent portion
of the West at the start of the period on Monday. Then a stronger
upper trough digging toward the West Coast, along with associated
low pressure/frontal system, will support a plume of enhanced
moisture and yet another Atmospheric River event into parts of
California and the southwestern U.S. around Tuesday. Expect a
period of moderate to heavy lower elevation/coastal rain and heavy
high elevation snow, especially in the Sierra Nevada. Rainfall
totals should be less extreme than this past week's events while
snow levels remain lower, somewhat reducing the flooding/runoff
risk in comparison. However, locally higher forecast rainfall
totals along with existing wet ground conditions support a
Marginal Risk in the day 5 (12Z Tuesday-12Z Wednesday) Excessive
Rainfall Outlook for areas from the lower elevations of the Sierra
Nevada to the southern California coastal ranges, as well as
favored terrain over central Arizona. Eastward progression of
this system should then bring significant snowfall into parts of
the Great Basin and central Rockies Monday-Wednesday. The heaviest
precipitation overall should stay across the central/southern half
of the West, with lighter precipitation chances to the north.
This system should then emerge over the Plains and Great Lakes by
Thursday-Friday, with some snow possible over far northern areas
and rain/thunderstorms over the eastern Plains/east-central U.S.
Currently expect the heaviest rainfall to extend from the ArkLaTex
region into the Lower Great Lakes starting on Thursday. There has
been quite a bit of uncertainty and guidance changes as to the
axis of heaviest rainfall, but there continues to be a good signal
for the potential of heavy rain totals, possibly widespread.
Severe weather potential should also ramp up across the region.
Light to moderate rain will begin to spread into portions of the
Mid-Atlantic/Northeast by Friday, with the chance some light snow
may mix in over the interior Northeast. Another Pacific system may
bring rain and mountain snow to the Pacific Northwest and parts of
California toward the end of the week.
The early part of the week will see well below normal temperatures
across the southern tier with some localized readings of 20F or so
below normal. Near or below freezing lows over some areas may be
harmful to any new spring vegetation. There will be a steady
warming trend over the eastern half of the country to above normal
temperatures by the latter half of the week, ahead of the system
tracking out of the West. The mean trough aloft over the West
will maintain a broad area of below normal highs across that
region, with minus 10-20F anomalies most common Tuesday onward.
The far northern Plains will also see below normal temperatures
for most of next week, especially on Monday-Tuesday.
Putnam/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, 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