Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
416 PM EDT Tue Jun 22 2021

Valid 00Z Wed Jun 23 2021 - 00Z Fri Jun 25 2021

...A Slight Risk of excessive rainfall extending from the southern
Mid-Atlantic to the Central Gulf Coast will remain in effect overnight...

...Severe thunderstorms will be possible this over the Central Plains and
the Middle Mississippi Valley through Thursday...

...A Critical Risk of fire weather conditions has been issued for portions
of the Central Great Basin...

A long, slow moving frontal boundary traversing the coastline from the
Northeast to Gulf Coast and into the Southern Plains will be the focal
point for heavy rainfall for much of the short-range period. High moisture
values ahead of the front mixing with a destabilized atmosphere will
likely result in the development of showers and thunderstorms with high
rain rates that could produce isolated to scattered flash flooding. At
present, a Slight Risk of excessive rainfall has been issued by the
Weather Prediction Center from the Mid-Atlantic to the central Gulf Coast
and will remain in effect through Wednesday morning. While the northern
half of the frontal boundary is forecast to propagate offshore by
Wednesday morning, the southern portion over the Southeast and Gulf Coast
is expected to stall and linger. With the front sitting in place
convection will continue to focus over these regions, extending the risk
of isolated flash flooding for some areas through Thursday morning.

Elsewhere, showers and thunderstorms are forecast to develop along a weak
quasi-stationary front extending from the Central Plains to the Middle
Mississippi Valley. Due to the potential for some of these storms to
become severe and capable of producing large hail and gusty winds, the
Storm Prediction Center has issued a Slight Risk of severe thunderstorms
for portions of Iowa and eastern Nebraska through Wednesday morning.
Isolated flash flooding associated with these storms will also be a
concern.

By Wednesday evening, a cold front diving south through central Canada
will reach the Northern Plains and Upper Midwest, triggering showers and
thunderstorms throughout the regions. Similarly to the previous day,
damaging winds and large hail could be produced by these thunderstorms if
they were to become severe. As such, the Storm Prediction Center has
issued a Slight Risk of severe thunderstorms for much of Nebraska, with
Marginal Risks issued for portions of the Central Plains and Upper
Midwest. Heavy rainfall associated with these storms will also create a
risk for scattered flash flooding.

Out West warm, dry, and windy surface conditions in the Central Great
Basin and Southwest will create favorable conditions for fire weather,
with a Critical Risk area encompassing much of central/southern Utah,
eastern Nevada, and northwestern Arizona. Additionally, dry thunderstorms
with isolated lightning strikes will be possible over the Northern Great
Basin where dry fuels are present, creating another risk area for fire
weather. With these conditions in place, Red Flag Warnings are currently
in effect for portions of the Southwest, Great Basin, and Central Rockies.
To the south, a plume of moisture surging into the Southwest and Southern
California will interact with upper-level energy to trigger showers and
thunderstorms over the regions Wednesday morning before spreading into
parts of the Great Basin and the Central/Southern Rockies overnight.

Zavadoff
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
404 PM EDT Tue Jun 22 2021

Valid 12Z Fri Jun 25 2021 - 12Z Tue Jun 29 2021

...Dangerous Heat Wave to build record temperatures across the
Northwest/West...
...Elongated Heavy Rain Threat to stretch from Southern Plains to
Great Lakes...

19Z Update: There has been a general westward shift in the heavy
rainfall axis from the Central Plains to the Great Lakes region
with the 12Z guidance compared to the 00Z guidance. This is in
response to a stronger western Atlantic upper ridge that will slow
the progress of the cold front and the shortwave impulses along
it. The 12Z CMC was well to the east of the GFS/ECMWF and their
respective ensemble means regarding the placement of both the
central U.S. upper trough and also with the Atlantic ridge, so it
was not incorporated in this forecast cycle. Otherwise, there is
generally above average forecast confidence through Saturday on
the overall synoptic scale set-up. The upcoming heat wave for
much of the Intermountain West and interior portions of the West
Coast states is also on track to make weather headlines, with the
13Z NBM appearing reasonable as a starting point in the forecast
process. Numerous record high temperatures will likely be
established, and potentially some monthly June records could be
within reach, particularly across portions of Washington and
Oregon where afternoon highs could reach or exceed 110 degrees for
some lower elevation interior locations! The previous discussion
is appended below. /Hamrick


...Guidance/Predictability Evaluation...

The WPC medium range product suite was primarily derived from a
composite of best clustered solutions of the 18 UTC GFS/GEFS mean
and 12 UTC UKMET/ECMWF/ECENS mean Friday into Saturday in a
pattern with above average continuity and predictability.

The models exhibit considerable spread as well as run-to-run
variability over the weekend with and downstream from a Rex
blocking pattern on the West Coast. The ensemble means remain
compatible through early next week and were used in composite with
similarly consistent guidance from WPC continuity and the 01 UTC
National Blend of Models. This composite develops and maintains
north-central U.S. upper troughing a bit on the amplified side of
the full envelope of guidance, seemingly consistent with upstream
blocking. Newer 00 UTC guidance overall continues these general
trends.

...Weather Highlights/Hazards...

A highly anomalous upper ridge will build inland over the
Northwest by the weekend and the heat wave will expand for much of
the West into next week. Temperatures will climb to 15-25+ degrees
above normal from Washington/Oregon to the Northern Rockies, and
more generally 5-15 degrees above normal along and west of the
Rockies. Records are likely from the Pacific Northwest to the
north/central Great Basin and northern Rockies, with temperatures
peaking over 100F to near 110F forecast for interior sections of
the West. Little to no rainfall is expected to ease this excessive
heat.

Downstream, robust upper trough digging from south-central Canada
will solidify a wavy, fortified and slow moving cold front to
stretch from the south-central to northeastern U.S. this weekend
and early next week, inviting deep moisture return and pooling.
Well organized showers and storms will significantly increase
rainfall and runoff potential along and ahead of the front with an
elongated and focused zone from the Southern Plains northeastward
across the Midwest/Mid-South, Ohio Valley and Appalachians to the
Northeast. Episodes of cell training and flooding are likely
across some of these areas.

Schichtel