Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Mon Jul 08 2019
Valid 12Z Thu Jul 11 2019 - 12Z Mon Jul 15 2019
...Overview and Guidance/Predictability Assessment...
Models and ensemble means show a fairly strong upper ridge
centered over the southern Rockies/Four Corners region through the
period, possibly extending into parts of the central-southern
Plains at times. Farther west a mean trough will persist over the
eastern Pacific, with individual ejecting shortwaves likely
becoming embedded within progressive flow across the northern tier
states/southern Canada before reaching a shallow mean trough over
the Northeast. Some of these shortwaves will have low
predictability but overall the most troublesome part of the
forecast continues to involve low pressure expected to be over the
northern Gulf of Mexico as of the start of the period.
Uncertainties over the degree of development and fine details of
surrounding flow lead to well below average confidence in the
ultimate path of the system and associated sensible weather
effects.
Consensus and continuity still look good for the shortwave
crossing the Great Lakes/Northeast Thu-Fri and associated surface
low/trailing front. The 12Z CMC offered an extreme solution
regarding how much energy in the southern periphery of the trough
might reach the Southeast (affecting the Gulf low). Guidance is
starting to maintain better definition beyond Fri for an upstream
wave tracking across southern Canada and trailing front crossing
the northern U.S.. Thus confidence is now sufficient to continue
eastward progress of a front reaching the northern Plains by Fri,
crossing the Great Lakes and Northeast/Mid-Atlantic Sat into early
Mon.
For the eastern Pacific trough the ensemble means have nudged
their forecasts slightly eastward over the past day, partially
reflecting some operational model solutions that have been
bringing one or more stronger shortwaves into the Pacific
Northwest. Model runs have been erratic with these shortwaves
though, not surprising given the features' small scale. This
favors a model/mean blend as the best starting point.
Guidance available through the 18Z cycle did not really provide
any improvement in confidence for the track of the possible Gulf
system. The 12Z CMC (due in part to its aforementioned issue with
northern stream energy) and 12Z GFS offered northeastward tracks
while recent ECMWF runs and 18Z GFS would have the system crossing
the central/east-central Gulf Coast and continue northward from
there. These solutions are within an even broader ensemble
envelope. A key part of the forecast appears to be exactly how
much upper ridging will exist between the Gulf system and the
northern stream shortwave late in the week. It is hoped that this
can be resolved fairly soon and help to narrow the guidance
spread. At the very least the new 00Z guidance seems to be
reducing the probability of the northeastward scenario as the
GFS/ECMWF have trended westward and the CMC is taking longer
before exhibiting its consistent northeast tendency. The manual
forecast remains close to yesterday's coordinated track which is a
little east of the 18Z GFS.
After a model-dominated blend to start day 3 Thu, the forecast
transitioned toward half to two-thirds combined inclusion of the
18Z GFS/12Z GEFS mean which were closest to the desired track for
the Gulf system. The remainder went to the 12Z ECMWF mean and
past two ECMWF runs. The minimal ECMWF input went more toward the
old 00Z/07 run which was a little closer to preference than the
12Z version. This blend provided a good representation of the
model/ensemble compromise for the eastern Pacific trough and
waves/frontal systems crossing southern Canada and the lower 48.
...Weather Highlights/Threats...
The heavy rainfall threat continues for some areas along and
inland from the Gulf Coast, with specifics depending on the very
uncertain path of the system expected to track over a portion of
the northern Gulf. Given the diverse guidance spread thus far, it
is important to note that a meaningful potential exists over a
greater area than represented on any single deterministic model or
manual forecast. Latest trends are suggesting there may be a
westward shift in highest rainfall potential but further runs will
be needed for confirmation. Elsewhere, low pressure and trailing
front will bring showers/thunderstorms to the Northeast and
Mid-Atlantic late this week with best dynamics aloft supporting
highest rainfall totals over parts of the Northeast. Some
showers/storms may also accompany another frontal system tracking
from the northern Plains into the East late week through the
weekend. The trailing part of the front may stall over the
northern Plains to provide a longer-term rainfall focus.
Locations along and just inland from the Pacific Northwest coast
may see one or more periods of scattered light rainfall depending
on strength of shortwaves ejecting from the mean trough aloft over
the eastern Pacific. Portions of the Rockies may see scattered
diurnal convection on one or more days.
Most temperature anomalies should be in the single digits through
the period. Aided by warm morning lows in particular, areas from
the interior West through the northern Plains should be
consistently above normal. Frontal progression will provide some
temperature variability over the northeastern quadrant of the
lower 48. Clouds/rainfall may keep high temperatures a little
below normal over some areas along and inland from the Gulf Coast,
depending on the track of the possible Gulf system.
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