Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1152 AM EDT Sat Sep 21 2019
Valid 12Z Tue Sep 24 2019 - 12Z Sat Sep 28 2019
...Heavy rainfall threat over the Southwest decreases after Tue...
...Areas of heavy rain possible over parts of the central U.S.
during the period...
...Rain and mountain snow expanding over the West late
...Expanding area of record highs in the Southeast late next week
through next weekend...
00Z/06Z guidance showed very good clustering for the first few
days of the forecast (through about --------) before differences
in the Southwest greatly increased. Though the 06Z GFS and 00Z
UKMET/Canadian shifted quicker than earlier runs, the ECMWF and
its ensembles remained consistent from before. The 00Z GFS was
only somewhat quicker than the ECMWF but the GEFS members were
quicker as well. Given the multi-day waffling in much of the
non-ECMWF guidance, opted to stay near the more steadfast cluster
of solutions that center near the ECMWF/ECMWF ensemble mean and
continuity. Thereafter, still expect the closed low to weaken and
lift northeastward through the southern Plains as much deeper
troughing digs down through the West later in the week. Guidance
was in excellent agreement on this amplified longwave pattern
(much better than average agreement) but showed differences in
downstream (i.e., central CONUS) details with the surface
boundaries/lows. Thus, opted to rely mostly on the 00Z ECMWF
ensemble mean and NAEFS mean along with continuity and some 00Z
ECMWF as it was near its mean.
Previous discussion from 0659Z is below
Guidance continues to show a brief period of fairly flat mean flow
followed by a rapid transition to a very amplified West Coast
trough/eastern ridge by next weekend. As this pattern becomes
established expect a pronounced trend to much below normal highs
over the West with increasing coverage of precipitation including
higher elevation snow. Meanwhile the eastern half of the country
should see an expanding area of much above normal temperatures.
Between these two air masses, portions of the Plains/Mississippi
Valley will be in the best position to see one or more episodes of
locally heavy rainfall. Guidance continues to have difficulty in
resolving specifics of an upper low reaching/stalling in the
Southwest by early Tue and timing of northeastward ejection ahead
of the amplifying West Coast trough. This feature and leading
moisture from the tropical East Pacific may support some enhanced
rainfall near the start of the period and eventually over the
central part of the country.
The early part of the updated forecast started with highest weight
on the 12Z ECMWF with supporting contributions from the 12Z
GFS/UKMET/CMC. As the amplified trough-ridge pattern becomes
established the forecast transitioned to greater emphasis on the
past two ECMWF runs (to downplay detail uncertainties) and ECMWF
mean with the remainder composed of the 12Z CMC and 18Z GEFS mean.
Guidance still varies for some aspects of the upper trough/low
affecting New England Tue-Wed. At the very least the 18Z GFS had
the lowest probability solution with a progressive open shortwave
versus slower upper low. The new 00Z GFS improved upon that 18Z
run but is still on the faster side of the envelope. Farther west
a blend provided a reasonable starting point for troughing that
reaches central Canada and the northern Plains by Wed. One trend
of note is that consensus has adjusted farther south with the
associated central-eastern U.S. front. Meanwhile guidance
continues to vary among each other and run to run with the
Southwest upper low. The manual forecast incorporated the slower
tendency of most 12Z models but confidence is still lower than
desired as some of the new 00Z runs have jumped faster. Either
way the amplifying western trough will at some point promote rapid
ejection of the upper low.
By late Wed or Thu through the rest of the week there are still
some pronounced differences for individual pieces of energy
arriving into the Northwest as the upper trough begins to take
shape. In varied ways recent GFS runs have differed from
consensus enough to downplay in the forecast blend after day 5
Thu, and in addition lower heights into the Plains somewhat more
than consensus late in the period. ECMWF/GEFS means have been
fairly stable and similar with the trough into day 7 Sat while the
ECMWF runs back to the 12Z/19 cycle have been stable with the
overall trough and Pacific Northwest upper low. What minor trends
existed through the 12Z cycle seemed to favor hedging a little
closer to the coast and the new 00Z CMC has adjusted to this
scenario as well. Fast southwesterly flow downstream will present
shortwave challenges (including the ejecting Southwest upper low)
so it will likely take some time to resolve specifics of central
The upper trough amplifying along or near the West Coast late this
week will lead to increasing coverage of precipitation over
portions of the West. Best potential for high elevation snow
should be over the extreme northern Rockies with lesser
probabilities over the Northwest and perhaps even into the Sierra
Nevada by next weekend. Medium/smaller scale detail uncertainties
aloft temper confidence in where heaviest precip will occur but
potential exists for some enhancement over favored terrain from
the Pacific Northwest/northern California into the northern
Rockies/Great Basin. Farther south expect the compact upper low
initially over the Southwest to support some localized rainfall
enhancement early in the period.
Locations along and near an axis from the central Plains into
Upper Mississippi Valley will see the best potential for one or
more episodes of heavy rainfall during the period. Once the
amplified upper trough-ridge pattern gets established,
teleconnections suggest the best emphasis should be over the Upper
Mississippi Valley. One convective episode may accompany a cold
front approaching the region early in the period along with a
leading advancing warm front. The cold front will stall to the
south and then return back to the north with one or more waves,
providing a focus for additional convection. One or more
shortwaves aloft and eastern Pacific tropical moisture may
contribute as well.
The cooling trend over the West will start over northern areas
with the Northwest seeing highs 10F or more below normal by Thu.
Minus 10-20F (and locally greater) anomalies for highs should
cover an increasing proportion of the West by Fri-Sat.
Clouds/precip will keep min temps from being nearly as extreme.
At the same time temperatures 10-20F above normal will become more
common over the eastern half of the country by late week and the
weekend. Daily record highs appear increasingly likely over and
near the Southeast from Thu onward, quite possibly past the medium
range period as well given the CPC day 6-10 and 8-14 day outlooks.
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at: