Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Mon Aug 19 2019
Valid 12Z Thu Aug 22 2019 - 12Z Mon Aug 26 2019
...Overview...
From late this week into the weekend expect an upper low crossing
eastern Canada to anchor a trough that crosses the Great
Lakes/Northeast. The trough should depart after Sat. The leading
cold front reaching the East as of early Thu will likely stall
over the Southeast while the trailing part of the front over the
Plains returns as a warm front (though with slower multi-day
trends). To the west low amplitude mean flow should prevail from
the North Pacific into western-central parts of the northern
CONUS/southern Canada, with a trend toward some troughing by
Sun-Mon. Specifics will depend on the yet-to-be determined
relative strength and position of ridging over the eastern Pacific
and southern half of the western U.S.. Individual shortwaves
within the flow will bring at least a couple fronts into the
northwestern states and northern Plains. Some guidance still
hints at energy aloft lifting across the extreme western Gulf of
Mexico toward the Gulf Coast.
...Guidance/Predictability Assessment...
For the northeastern upper trough there is decent consensus in
principle. However shortwave differences between models and in
consecutive runs lead to some spread/variability in specifics of
low pressure forecast to track just north of New England and the
trailing wavy front that sinks through the Mid-Atlantic into the
Southeast. Meanwhile guidance is having a difficult time
resolving the details of shortwave energy and associated surface
reflection reaching western North America by the start of the
forecast early Thu and continuing east/northeast thereafter. The
primary forecast consideration in the larger scale was to discount
the 12Z GFS due to effects from a disagreeable short-range
evolution over the eastern Pacific extending into the western
states.
There is some relative agreement that the next Pacific shortwave
should reach the coast around late Fri-early Sat. Beyond that
time solutions have differed regarding progression of this
incoming energy as well as for strength of ridging over the
southern half of the West and over the eastern Pacific. There has
been a modest slower trend in consensus for the evolving northern
stream trough over the past couple days but latest ECMWF/CMC
ensemble means have been more stable with their depiction of the
trough relative to the latest GEFS mean runs that are somewhat
slower (corresponding to a weaker western U.S./eastern Pacific
ridge). Recent ECMWF runs and the 12Z/18Z GFS also support the
faster scenario. Finally, teleconnections relative to the core of
positive height anomalies expected just off the West Coast late in
the period favor a downstream trough far enough east to favor the
majority non-GEFS cluster.
From Thu into Sat a blend of mostly operational guidance
represented the most common ideas over the East while it provided
an intermediate approach and reasonable continuity aloft over the
Northwest into northern Plains. Some surface details differed
though. The 12Z NAEFS/ECMWF means balanced the 12Z ECMWF/18Z GFS
later in the period, with the NAEFS preferred for its inclusion of
CMC ensembles and only partial account for the GEFS.
...Weather Highlights/Threats...
The front dropping through the Mid-Atlantic into the Southeast and
extending back into the Plains continues to serve as a likely
focus for areas of heavy rainfall, with best potential for highest
totals along and just inland from the southern
Mid-Atlantic/Southeast coast as well as parts of the
central/south-central Plains into Mississippi Valley. The central
and possibly western Gulf Coast region may see some enhanced
rainfall from western Gulf energy aloft and possible associated
surface trough. Confidence is below average due to the small
scale of feature(s) involved. Expect scattered
showers/thunderstorms on a daily basis over the rest of the
Southeast/Gulf Coast region. Areas from the northern Plains east
into the Mississippi Valley may see one or more episodes of
convection with fronts reaching the region. It will take added
time to resolve enough of the specifics at the surface and aloft
to gain more confidence in pinpointing location/timing of any
enhanced rainfall.
Expect above normal temperatures to be most persistent from
southern Oregon and the northern two-thirds of California into the
southern High Plains with upper ridging in place during the
period. Scattered locations within this area may see max and/or
min temperatures at least 10F above normal. Frontal passages will
lead to more variation over northern parts of the West.
Central/eastern states should tend to see moderately below normal
to slightly above normal readings, with the central/northern
Plains into Great Lakes seeing the best opportunity for below
normal highs from Thu into Sat.
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