Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Mar 20 2023

Valid 12Z Thu Mar 23 2023 - 12Z Mon Mar 27 2023

...Heavy rainfall threat over parts of the east-central U.S. after
midweek...

...Overview and Guidance/Predictability Assessment...

Models and ensembles agree on an active pattern with a series of
moderately progressive systems affecting the lower 48, while a
slower-evolving upper trough/surface system amplifies just off the
West Coast by next Sunday-Monday.  Upper ridging centered over the
Gulf of Mexico as of Thursday should progress eastward for a while
but could rebuild back to the west-southwest by the start of next
week.  The most prominent focus of this forecast period will be on
a heavy rainfall event forecast over the east-central U.S. around
Thursday-Friday as an upper trough emerges from the West and leads
to abundant moisture interacting with a wavy front trailing from a
northeastern U.S./southeastern Canada system, as well as the
wave/front supported by the western dynamics.  The Storm
Prediction Center is also monitoring severe weather potential over
some areas from the southern Plains eastward.  The Interior
Northeast could see meaningful snow from this system by Saturday. 
Meanwhile, areas from the northern/central West Coast into the
Rockies should see periods of light to moderate rain/mountain snow
late this week into the weekend.  Then the upper trough expected
to develop just off the West Coast late in the period may lead to
higher totals over some coastal areas by the start of next week,
depending on exact details of the upper trough.

Guidance clustering has been gradually improving for the system
crossing the northeastern U.S./southeastern Canada Thursday into
early Friday.  Latest GFS runs are most at odds with other
solutions by straying to the fast side with leading shortwave
energy that supports the wave tracking east of northern New
England by Friday.  Including the 00Z version, latest GEFS means
offer much better support for the non-GFS cluster for the
specifics of this system.  Confidence is low with specifics of
possible weak waviness along the trailing front over the East into
Friday.

The past 12 hours of model runs are finally attempting to converge
regarding the details of the upper shortwave tracking out of the
West and through the Plains, along with the associated low
pressure evolution.  The prior wide array of guidance is generally
gravitating toward recent ECMWF mean runs that have been depicting
the surface system reaching the vicinity of the Ohio Valley by 12Z
Saturday and near the New England coast or Canadian Maritimes 24
hours later, corresponding to some sharpening and possible closing
of shortwave energy.  Operational ECMWF runs have been a bit on
the northwestern side of the spread at the surface.  Having the
current/recent solutions finally look more similar is definitely
progress, though exact track is still an issue and quite a bit of
spread still exists for how strong the system will be by the time
it affects New England.  From the 12Z/18Z cycles that were used
for the updated forecast, the main consideration was to lean away
from the more suppressed 12Z UKMET by the end of its run early
Saturday.

Recent model runs have been waffling with respect to the character
of upstream coming into the West late this week into the weekend,
though latest consensus has returned to a fairly broad and diffuse
presentation which has been most persistent in the ensemble means
and in half or more operational models from prior cycles.  As this
energy continues east, the means agree quite well on the shortwave
reaching near the Mississippi Valley by early day 7 Monday with
low pressure over the Great Lakes.  Operational model runs,
especially the GFS, have been inconsistent with the strength and
other details of this system.  This favors a blended/ensemble mean
approach for the time being.

There are still plenty of detail uncertainties for the upper
trough expected to amplify just off the West Coast by
Sunday-Monday.  Most guidance including the ensemble means
continue to advertise a fairly clean departure of leading energy
followed by amplification of separate energy dropping southeast
from Alaska, while ECMWF runs still hold onto more of the leading
energy and have some form of interaction with the Alaska energy. 
Thus far the ensemble means have been providing the best overall
template, showing the upper low reaching just off the Pacific
Northwest coast by next Monday.  The new 00Z CMC has adjusted to
this idea too after being more along the coast in the prior run. 
GFS runs are tending to pull the upper low a bit westward of
consensus, leading to an overall trough somewhat west of what
teleconnections would recommend based on the location of strongest
positive height anomalies upstream over the North Pacific.  While
the ECMWF details are still a question mark, by the end of the
period the 12Z run does end up getting closer to the preferred
means.

Based on above considerations, the updated forecast based on
12Z/18Z guidance started with an operational model blend but with
some split of 18Z GFS/GEFS mean input.  Then by late in the period
the blend rapidly trended toward increasing 18Z GEFS/12Z ECMWF
mean guidance with minority remnants of GFS/ECMWF ideas.

...Weather/Hazards Highlights...

The leading system crossing the northeastern U.S. and southeastern
Canada will spread some light to moderate precipitation across
parts of the Northeast, including some snow over northern New
England.  Then the focus will turn to a potential heavy rain event
over the east-central U.S. during Thursday-Friday, as an upper
trough initially over the West progresses eastward while
already-established moist flow from the Gulf of Mexico persists
into late week and interacts with a wavy front left behind by the
Northeast system.  Even with wide array of solutions for specifics
at the surface and aloft over recent days, there was a consistent
signal for this event in principle, and now some of these details
are starting to get better refined--but some additional
adjustments are still likely.  The new day 4 (12Z Thursday-12Z
Friday) Excessive Rainfall Outlook will be quite similar to the
previous day 5 issuance, reflecting only minor adjustments for
latest guidance with respect for a Slight Risk area from eastern
Oklahoma and vicinity northeast into western Ohio.  The day 5 (12Z
Friday-12Z Saturday) will follow along with the southeastward
progression of heaviest rainfall, with a Slight Risk area from
parts of Arkansas/northern Louisiana through much of Kentucky. 
Strengthening of low pressure later in the day 5 period should
eventually lead to some acceleration of rainfall by early
Saturday.  Continue to monitor Storm Prediction Center outlooks
for the latest information regarding severe weather threats from
the southern Plains eastward.  As the system continues
northeastward, interior portions of the Northeast may see
meaningful snowfall depending on the exact low track. 

The initial upper trough crossing the West followed by upstream
energy will support periods of rain and mountain snow from the
northern/central West Coast into the Rockies.  Most amounts should
be in the light to moderate range but localized enhancement will
be possible.  Once this trailing energy departs from the West,
another system may bring some precipitation of varying intensity
to portions of the central/eastern U.S. by Sunday-Monday.  Also in
that time frame, the upper trough most likely amplifying just off
the West Coast may start to increase precipitation totals along
the central coastal areas based on the consensus trough position.

The large scale pattern will keep much of the West and parts of
the northern Plains quite chilly through the period, with broad
coverage of highs 10-20F below normal each day.  The Great Basin
and central Rockies could see highs as cold as 20-25F below normal
during the weekend.  Some record cold highs may be possible. 
Temperatures over the Northwest should be less extreme but still
be mostly below normal.  In contrast, an area of well above normal
temperatures will extend from the central-southern Plains through
much of the East during the latter half of the week. 
Thursday-Friday will be the warmest days for highs with plus
10-20F anomalies and some morning lows that could be as much as
20-25F above normal Thursday into early Saturday. Frontal passage
across these areas will bring a cooling trend to near or slightly
above normal levels.

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