Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
247 AM EDT Sun Jun 24 2018
Valid 12Z Wed Jun 27 2018 - 12Z Sun Jul 01 2018
...Overview and guidance/predictability assessment...
Latest guidance continues to show the mean pattern transitioning
toward an amplified western ridge/eastern trough configuration. A
notable trend over the past day has been toward greater northward
amplitude of the eastern ridge by next weekend, with the center of
the embedded upper high possibly drifting at least as far north as
40N latitude. This evolution would direct highest temperature
anomalies originating from the central Plains/mid-upper MS Valley
into the Northeast. The primary uncertainty with the western
trough aloft is for its depth and the details of individual
shortwaves. The latter will affect the forecast for what may be a
convectively active period across the Northern Tier states.
For the western trough, GFS/GEFS runs have been a bit on the
stronger and deeper side of the spectrum but worthy of partial
inclusion in the forecast given the strength of the upper ridge
building over the eastern Pacific. Meanwhile a greater proportion
of ECMWF/CMC runs and their ensembles suggest a somewhat more
shallow overall trough due to faster ejection of leading energy in
response to upstream flow--which is also plausible within the
context of a multi-day mean trough. However recent ECMWF runs
have been inconsistent with the evolution of trough energy by next
weekend. In new 00Z guidance thus far the GFS has trended toward
a 12Z ECMWF type of ejection late in the period albeit with
differing upstream details, while the CMC maintains a broader mean
trough on most days. Potential merit for varying clusters and
GFS/ECMWF run to run variability favor a compromise approach for a
single deterministic forecast.
Farther east the best-behaved part of the forecast is for the
system expected to be over the Great Lakes as of day 3 Wed.
Guidance over recent days has tended to show moderate spread with
a consensus approach maintaining good continuity. In the wake of
this feature, as previously noted the recent trend in most
guidance is for eastern ridging to extend farther northward. By
day 7 Sun the past couple ECMWF runs show the farthest north
position for the upper high, reaching New England.
Teleconnections relative to the upstream pattern offer potential
for somewhat flatter flow than advertised by the
strongest/northern solutions with the eastern ridge, leaving a
model/mean blend as a reasonable starting point late in the
period. Guidance is suggesting development of a weakness aloft
over the Southeast/Florida with possible surface reflection in the
form of a trough. Predictability for specifics appears to be
below average given diffuse nature of the feature.
A blend among the 18Z GFS and 12Z ECMWF/UKMET/CMC represented
consensus well during the first half of the period while guidance
spread/variability favored a trend toward a more even
model/ensemble weight by the weekend. The 18Z GEFS/12Z ECMWF mean
provided the ensemble input for days 6-7 Sat-Sun while operational
ECMWF variability led to including a small component of its past
two runs by that time frame. The blend held onto modest weight of
18Z GFS/12Z CMC runs as well.
...Weather threats/highlights...
The system initially over the Great Lakes will likely support some
areas of moderate to heavy rainfall early in the period, most
likely from eastern Great Lakes/upper Ohio Valley into
western-northern New England. As the upper pattern becomes more
amplified and a mean frontal boundary/low pressure persist over
the Plains and vicinity, there will be the potential for multiple
convective episodes and accompanying heavy rainfall from over or
just east of the northern Rockies into the upper MS Valley/upper
Great Lakes. Specifics will depend on less predictable details of
shortwaves ejecting from the western trough aloft. Currently the
best signal for highest 5-day rainfall totals exists over the
upper MS Valley/upper Great Lakes. With lighter amounts, the
western upper trough will bring scattered rainfall to other areas
within the northern half of the region. The Southeast/Florida
will see areas of locally heavy convection during the period.
Enhancement to rainfall may come from a weakening front early in
the period and then the developing upper weakness/surface trough.
An expanding area of hot/humid conditions will likely cover a
large portion of the central-eastern U.S. by late this week and
weekend. Currently expect the axis of highest min/max temperature
anomalies of plus 10-20F to extend from the central Plains through
the Great Lakes/Ohio Valley and into the Northeast. Any daily
records appear to be more likely for warm lows than daytime highs.
Increased low-level moisture will also lead to very high heat
index values over many areas. On the other hand the upper trough
amplifying into the West will bring a pronounced cooling trend.
Locations in the northern Rockies/High Plains should see highs as
much as 10-15F below normal by Fri-Sun with less extreme anomalies
elsewhere.
Rausch
WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities are found at:
http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4