Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Fri Jun 07 2019
Valid 12Z Mon Jun 10 2019 - 12Z Fri Jun 14 2019
...One or more episodes of excessive rainfall and flash
flood/flooding threats possible over parts of the
Appalachians/East Coast states next week...
...Overview and Guidance/Predictability Assessment...
During Mon-Wed guidance agrees in principle that initially weak
Pacific shortwave energy reaching western North America by early
Mon will steadily amplify toward the east-central U.S. as a ridge
builds into the western part of the continent. This evolution
will occur behind a couple leading features (a Southeast
U.S./eastern Gulf trough and separate northern stream shortwave).
The overall combination will lead to one or more periods of
enhanced rainfall over some areas of the East. By Thu-Fri details
aloft over much of the Lower 48 become increasingly uncertain but
most models/ensembles show a general theme of diffuse shortwave
energy moving into the West while the amplified eastern trough
that persists into Thu becomes more shallow on Fri.
From day 3 Mon into day 5 Wed an operational model blend (using
the 18Z GFS/12Z ECMWF and lesser weights of the 12Z UKMET/CMC)
represents the synoptic consensus well, while yielding an
intermediate solution for important smaller scale differences that
are within typical error ranges--and have below average
predictability--for the time frame of interest. In particular
some of these differences affect the location/intensity/duration
of heavy rainfall over the East. Broadly speaking most models and
ensemble means have shown fairly stable evolutions over the past
24-36 hours with no pronounced trends evident.
During the latter half of the period there are multiple detail
questions that arise: whether the sharpening trough energy
reaching the Mississippi Valley around midweek closes off a low
for some period of time (a solution somewhat in the minority thus
far), the evolution of energy over Ontario (the 12Z ECMWF and to a
lesser degree ECMWF mean more amplified), and the particulars of
multiple pieces of energy that will comprise the overall diffuse
shortwave trough moving into the West. Adding 18Z GEFS/12Z ECMWF
mean guidance to the blend, reaching nearly half weight relative
to the operational models by day 7 Fri, helps to downplay the less
confident details within the more agreeable large scale evolution.
During this time frame the ECMWF component also includes the
prior 00Z run as its eastern solution becomes more compatible with
consensus than the 12Z run.
...Sensible Weather Highlights/Threats...
Expect highest rainfall totals for the full Mon-Fri period to be
over an area encompassing the eastern Great
Lakes/Appalachians/East Coast states. Early in the week a
north-south front heading into the East and wavy front over the
Southeast will provide focus for rainfall. The trailing front and
embedded wave(s) will likely stall over the Southeast Tue-Wed and
finally be dislodged as the upper trough amplifying into the East
brings another surface low/frontal system into and through the
region. Over the central third of the U.S. there will likely be
areas of showers/storms with some moderate to heavy amounts but on
a more localized scale. Low level upslope flow may enhance
rainfall over the southern High Plains early in the week and a
frontal system/low pressure may bring rainfall to portions of the
northern half of the Plains and points east from Mon night or Tue
through midweek. The shortwave energy moving into the West late
in the week may promote scattered convection with some activity
possibly reaching the Plains as well.
The upper ridge building into the West early-mid week will bring
very warm to hot temperatures to the region, with fairly broad
coverage of plus 10F and greater anomalies Mon-Wed. Readings will
begin to moderate later in the week as heights aloft decline.
Expect highest temperatures Tue-Wed with locations along the
Medford-Portland corridor seeing the best potential for highs 20F
or more above normal. Daily records for highs/warm lows will be
possible over the West Coast states. Meanwhile the pattern will
tend to favor below normal temperatures over most of the central
U.S. aside from a possible trend toward normal by Fri.
Clouds/rainfall should produce a decent area of minus 10-20F
anomalies for highs over the southern High Plains on Mon. The
East will see above normal mins early in the week but otherwise
should see near to below normal min/max readings as well.
Rausch
Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml