Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1157 AM EDT Sat Oct 12 2019
Valid 12Z Tue Oct 15 2019 - 12Z Sat Oct 19 2019
...Pattern Overview and Guidance/Uncertainty Assessment...
Guidance shows a fairly amplified but progressive pattern across
the CONUS during the extended period Tue-Sat. An amplifying
shortwave should cross the northern Plains/Midwest on Tue and the
Great Lakes/Northeast Wed-Thu. There has been decent agreement
that an embedded upper low will reach the vicinity of New England
by Thu and in fact a compact closed low could exist a day or two
before then. This feature will push a surface low and frontal
system across the eastern half of the nation. Most guidance has
been signaling formation of a new Mid-Atlantic wave on Wed and
intensification to a relatively deep cyclone near the New England
coast Wed night-Thu as the upper low intensifies. Farther west an
intense Pacific upper-level jet that will reach the Northwest
should carry a train of shortwaves, lowering heights across the
Northwest and eventually the northern Rockies as the series of
systems moves inland. By the end of the forecast period next Sat,
the models and individual ensemble members show a fair amount of
spread for details but over the past couple days there has been
decent consensus (especially among the ensemble means) toward an
amplifying trough over the West. Leading shortwave energy should
eject across the central U.S.
A compromise among latest guidance represents consensus well for
the system crossing the East and intensifying near New England,
with deepening ensemble mean trends over recent days providing
support for the fairly strong model solutions by early Thu.
Meanwhile over the past couple days guidance has essentially
fallen into two clusters for the western through central U.S.
shortwave during Thu-Sat, the faster and flatter GFS/UKMET versus
slower and sharper ECMWF/CMC. Respective ensemble means generally
lean toward their operational counterparts. Historical biases
favor maintaining a forecast that leans slower than the GFS
cluster. However there is some potential that the very fast
upstream Pacific flow could lead to somewhat faster timing than
the ECMWF. In addition, even with the different specifics the past
two days' worth of multi-day means from the various guidance end
up with a similar pattern. This suggests lower predictability for
the exact forecast of this feature. By next Sat the recent
agreement and consistency of the ensemble means for the
larger-scale amplifying western U.S. trough favor primary emphasis
on the means as a starting point. Recent GFS runs have been on the
deeper side of the spread with the trough while the 00Z CMC is a
bit on the slower side.
...Weather Highlights/Threats...
A stalled and potentially wavy front lingering over the Lower
Mississippi Valley to Southeast/far southern Mid-Atlantic on Tue
will be the focus for widespread showers and thunderstorms. Some
eastern Pacific tropical moisture may help to increase the heavy
rainfall potential over this region. A separate area of lighter
precipitation (mostly rain but perhaps a few pockets of snow) will
accompany the low pressure/front crossing the Upper Midwest and
Great Lakes. The northern system will pull the deeper moisture
over the South northward, resulting in widespread and locally
heavy rainfall over the Northeast as low pressure
consolidates/deepens. Highest elevations of northern New England
may see a little snow as the upper low passes through.
The series of systems affecting the Pacific Northwest will bring
an extended period of rain and some high elevation snow, with snow
levels expected to fall gradually through the week as lower
heights begin to press inland. The coastal ranges of the Northwest
and Cascades should see the heaviest precipitation with multi-inch
liquid equivalents possible for the five-day period.
By late next week or next weekend moisture should increase across
the central U.S. It remains a significant question mark how much
precipitation may occur with a front reaching the Plains on Fri,
with differences in shortwave amplitude/timing leading to a wide
spread in amounts. Currently the most probable scenario has fairly
light totals. The amplifying western trough aloft and
strengthening low level inflow from the western Gulf of Mexico by
next weekend should promote increased rainfall along and perhaps
north of the western-central Gulf Coast. A lingering surface front
over the Gulf of Mexico may also help to focus this activity.
Through Tue-Wed temperatures will be well below average across the
north central U.S. in the wake of the current winter storm. Highs
should be 15-25 deg F below average across portions of the
northern Plains and Upper Midwest before temperatures trend warmer
after midweek. Expect cooler temperatures to spread across the
eastern half of the nation by the mid-late week as Canadian high
pressure trails the system consolidating over New England by early
Thu. High temperatures 5-15 deg F below average are likely across
a large area from the Mississippi River to the Eastern Seaboard
Wed-Thu, before temperatures return to near seasonal norms by next
Fri-Sat. Below average temperatures (generally minus 5-10F
anomalies) will spread into the Pacific Northwest and northern
Great Basin by late next week as upper-level heights fall and the
series of frontal systems affect the region. The upper trough
amplifying over the West by next Sat should then increase coverage
of highs 10 deg F or more below normal.
Rausch/Ryan
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