Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
357 PM EDT Sun Sep 19 2021

Valid 00Z Mon Sep 20 2021 - 00Z Wed Sep 22 2021

...There is a Moderate Risk of excessive rainfall over parts of the
Tennessee Valley/Southeast through Monday morning...

...Temperatures will be 10 to 15 degrees below average over parts of the
Northern Rockies into the Northern Plains...

...There is an Elevated Risk of fire weather over parts of the western
third of the country through Monday morning...

An upper-level low over the Middle Mississippi and Western Ohio Valleys
and abundant moisture over the Southeast/Tennessee Valley into parts of
the Ohio Valley will produce showers and thunderstorms over the region
through Monday. Some of the thunderstorms will produce heavy rain.
Therefore, the WPC has issued a Moderate Risk of excessive rainfall over
parts of Tennessee Valley/Southeast through Monday morning. The associated
heavy rain will create numerous areas of flash flooding. Furthermore, many
streams may flood, potentially affecting larger rivers through Monday
morning. The excessive rainfall threat weakens slightly over parts of the
Tennessee Valley and the Southeast Coast on Monday into Tuesday morning.

Meanwhile, a front extending from the Upper Midwest roughly southwestward
to the Great Basin will move eastward to the Great Lakes and southward to
the Gulf Coast by Tuesday evening. Showers and thunderstorms will develop
along and ahead of the boundary starting overnight Sunday over parts of
the Northern/Central Plains that will move into the Upper Mississippi
Valley/Upper Great Lakes by Monday evening.

On Tuesday, the front will intersect the moisture over the Tennessee
Valley, increasing the threat of excessive rainfall over parts of the
Great Lakes and Ohio Valley. Furthermore, upper-level energy moving across
the Pacific Northwest into the Northern Rockies will produce rain with wet
snow at the highest elevations of the Northern Rockies/Northern
Intermountain Region ending by Monday evening.

Additionally, dry conditions and gusty winds across the West led to an
Elevated Risk of fire weather over parts of the western third of the
country through Monday morning. Therefore, Red Flag Warnings are out for
parts of California and Colorado. Moreover, temperatures will be 10 to 15
degrees below average over parts of the Northern Rockies into the Northern
Plains. Therefore, Freeze Watches are up over parts of Idaho and Freeze
Watches over parts of Colorado.

Ziegenfelder
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
243 PM EDT Sun Sep 19 2021

Valid 12Z Wed Sep 22 2021 - 12Z Sun Sep 26 2021

...Widespread Heavy Rainfall/Runoff Threat from the Great
Lakes/Ohio Valley to the Mid-Atlantic/Northeast Mid-Late Week...


...Weather Pattern Overview...

Guidance highlights a widespread heavy rainfall and runoff threat
with development of a closed upper low over the Ohio Valley and
Great Lakes region by Thursday that slowly lifts to the northeast
later week as an associated cold front underneath steadily tracks
across and offshore the East Coast. Upstream, another amplifying
upper trough and cold front is expected to dig southeastward from
western Canada into the north-central U.S./Midwest late week,
although not as strong and nearly as wet as the preceding system.
Meanwhile, modest upper trough digging down the West Coast and
back offshore California may focus lift/moisture to fuel local
showers mid-late week should be replaced by a warming upper ridge
to center over the Intermountain West into next weekend.

...Guidance and Predictability Assessment...

Model and ensemble forecast spread has decreased days 3-5
(Wed-Fri), bolstering forecast confidence to above normal levels.
The WPC medium range product suite was primarily derived from
composite blend of reasonably well clustered guidance from the 00
UTC ECMWF/UKMET/Canadian days 3-5 along with the 13 UTC National
Blend of Models (NBM). Uncertainty grows through next weekend, in
particular with the extent of amplification of an upper trough
slated to work into the north-central U.S./Midwest later week.
Model run to run continuity in this energetic northern stream flow
has been suspect, suggesting instead an ensemble forecast approach
with the generally compatible 06 UTC GEFS and 00 UTC ECMWF
ensemble means and the NBM. newer 12 UTC guidance is mixed, but
may overall offer better clustering with this stream. Out West,
the 06 UTC GFS offered a more outlier inland track of digging
upper trough energies that consensus over the West/Southwest, but
the 12 UTC GFS is now more in line with other models and ensembles
days 3-7.

...Weather/Hazard Highlights...

The mid-later week weather pattern will be highlighted by a
gradually progressive heavy rainfall and local runoff episode from
the Great Lakes/OH Valley to the Mid-Atlantic/Northeast Wed-Fri as
fueled by pooling moisture/instability with a developing a closed
upper low. The system will be accompanied by a strong cold front
that will progress across the eastern U.S. that will also provide
some focus for more modest rainfall down across the Southeast. The
lead large surface high and upper ridge that will be across the
eastern U.S. early in the week will eventually give way to the
approaching trough and front, which will herald a pattern change
compared to the recent extension of summer-like heat and humidity.
Steady and amplified upstream upper trough progression through
western Canada still in guidance Wed/Thu would also be primed as a
kicker to dig into an unsettled/cooled north-central U.S./Midwest
later next week along with another associated frontal system, but
with much less precipitation potential with depleted moisture
availability in the wake of the main lead system. Best potential
may over vicinity of the Great Lakes under main trough
position/possible closed low. Meanwhile, upper trough energy set
to reach the Northwest midweek is set to dig southward and
re-emerge over the eastern Pacific off CA offshore. This system
offers modest precipitation potential with track and with CA
proximity.

Schichtel