WPC Medium Range Archive
Medium Range Products Valid On:
(Day 7 PMDEPD: Valid 08/12/2018 to 08/16/2018)
Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1201 PM EDT Thu Aug 09 2018
Valid 12Z Sun Aug 12 2018 - 12Z Thu Aug 16 2018
...Pattern Overview and Guidance/Predictability Assessment...
The medium range period features a well forecasted upper closed
low temporarily meandering near the Ohio Valley Sun-Tue before
getting kicked northeastward by Canadian troughing late in the
week. A separated southwestward extension of the Ohio Valley low
will also linger across the Southern Plains region leading to a
heavy rain threat for Texas/Oklahoma this weekend and into early
next week. Out West, strong ridging should hold across the
Southwest, while possible weak shortwaves round the top of the
ridge across the Pacific Northwest and northern Intermountain West.
The latest models and ensembles show good clustering through day 5
for much of the CONUS. By days 6-7, normal run-to-run
discontinuities and timing/intensity differences arise,
particularly with the operational models. The one thing to note is
the 06z GFS looked quite different from its previous runs late in
the period, particularly out west, as it maintained strong riding
well into the Intermountain West. Its previous runs along with the
ECMWF, and to some extend the GEFS/EC Mean ensembles (albeit
somewhat washed out) indicate possible weak shortwave riding
overtop the ridge. For this reason, opted to use the 00z run of
the GFS rather than the 06z run to maintain a solution closer to
consensus, as well as continuity with showing some semblance of
weakness in the upper ridge across the West. This cycle of the
medr progs used a blend of the well clustered 00z ECMWF/GFS with
their respective ensemble means.
...Weather Highlights and Hazards...
A relief to the Western U.S. heat is finally in sight this weekend
across the Northwest and High Plains Sunday/Monday as upper
troughing and a surface cold front move through. The upper
trough/low over the Southern Plains will help fuel organized rain
and storms across Texas and westward into the Southwest/Four
Corners region, particularly along the favored terrain. Guidance
continues to waver on exact placement, but several inches of rain
are likely near the Red River Valley (west-central TX into OK)
during the Sunday to Thursday time frame. Rainfall lifts into the
Mississippi Valley and the Ohio Valley ahead of upper troughing
across the region later in the week. Monsoonal moisture and
possible locally heavy rainfall will continue across the Southwest.
A wavy front will linger across the U.S. southern tier during the
period which will help spark afternoon showers/storms and locally
heavy rainfall within the warm and humid airmass. As the Ohio
Valley upper low slowly lifts toward New England, more widespread
precipitation may be favored to its east side along the
Appalachians to the I-95 corridor where the ground remains quite
saturated in some places. This could develop into a heavy rainfall
threat considering the relatively slow system translation and
southerly fetch of ample moisture over the course of several days.
Summertime closed lows can be underestimated in terms of
precipitation intensity/amounts and longevity.
Another frontal boundary will drop out of Canada toward Montana
next Thursday or so with a return of 90+ degree temperatures
across the High Plains.
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at:
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