Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/20/19
AREA FORECAST DISCUSSION
National Weather Service Wilmington OH
946 PM EST Thu Dec 19 2019
Cold high pressure will continue to control regional weather
this evening through the overnight before giving way to
warming temperatures as the high exits further to the east and
upper level ridging begins to working in from the southwest.
Conditions will be mostly dry through the period.
.NEAR TERM /THROUGH FRIDAY/...
It is shaping up to be a very quiet overnight period across the
ILN FA with broad surface high pressure entrenched across the
eastern Ohio Valley. It will also be another cold night,
although not as cold as last night -- as lows will bottom out in
the lower 20s with the exception being where a more substantial
snowpack still resides from east-central IN through central Ohio
where some mid teens will be possible by daybreak.
Although there were some hints at the potential for some
patchy lower clouds or even some light fog to develop,
confidence is not overly high in this materializing and
therefore have kept the FA mainly clear through daybreak. Winds
will be light and variable with perhaps a light southerly
component at times.
Clear skies will continue for most areas this evening through
early Friday morning. There is some potential for increasing low
clouds and some fog to form toward daybreak north of Interstate
70 as a weak cold front heads east through the Great Lakes. This
showed up particularly well on the HREF and RAP earlier this
morning, and has now appeared on the 12Z NAM and GFS as well.
.SHORT TERM /FRIDAY NIGHT/...
On Friday, the weak boundary shifts to the east, leaving more
warm air advection regionwide. However, where snowpack and low
clouds linger across the north, highs will struggle to reach the
mid 30s. Elsewhere, highs will approach normal mid-December
Friday night, continuing light southerly winds will keep lows in
the mid 20s to around 30, or slightly above normal.
.LONG TERM /SATURDAY THROUGH THURSDAY/...
High confidence in a quiet period with temperatures averaging out
above normal, and without hazards from precipitation or wind
Saturday through Wednesday. Lower confidence creeps into the
forecast on Day 7 /Thursday/ with regards to rain chances.
Saturday-Wednesday - focus this period will be on upper trough and
closed low which will be in the Ark-La-Tex region on Saturday
morning. This system will move slowly east-southeast at it digs
into/along the Gulf Coast region by Sunday morning and then ejecting
east-northeast off the Georgia coast on Monday. There has been slow
model-creep northward in the past few days on the northern fringe of
light rain potential attendant to this system - from far southern
Tennessee several days ago into southern KY in recent runs. And
while the 19.12Z GEPS probabilities suggest some low-end rain
chances making it as far north as the Ohio River on Sunday night or
Monday morning, just not seeing similar agreement in 19.00Z EPS nor
19.12Z GEFS probabilities, nor are we seeing anything in the higher-
resolution deterministic or control runs for these ensembles. So
will be keeping the forecast area dry through the weekend with only
some increase in clouds as the cirrus shield tries to cover the
south/east portions of the forecast area on Sunday/Sunday night. To
support this, EPS/GEFS/GEPS ensemble mean PWATs are all well below
seasonal normals through this period in the subsidence/ridging
region on the north/west side of the cyclone.
On Tuesday, upper trough will shift into the Intermountain West
which forces more substantial and full-latitude ridging over the
eastern US. This will keep dry conditions over the forecast area
Tuesday through Wednesday. There`s pretty solid agreement /for Day 7
in December/ of energy ejecting out of the Desert Southwest and up
the western side of the southeast US ridge and then topping the
ridge through the Great Lakes on Thursday. All deterministic runs of
GFS/CMC/ECMWF bring a weak cold front through the Ohio Valley
sometime either Wed night or Thursday. Rain chances in question -
19.00Z EPS has very low probabilities for measurable precipitation,
with 19.12Z GEPS in the 20-40% range, and most of the GEFS members
showing a swath of light rain along the front. With NBM giving 20-
30% chances, was inclined to keep those for now as there`s enough
there to mention some small rain chances. Of concern is a PWAT
anomaly that is drying with time as the system crosses the Ohio
Valley, and a parent shortwave well to the north of the area,
relying only on frontal convergence to generate some rain, not to
mention the offshore trajectories in the Gulf for a while early next
week in the wake of the departing southeast US upper trough which
will delay moisture return to the system.
Temperatures will incrementally rise Saturday through Tuesday as +2
sigma 500mb height anomalies combine with 850mb temp anomalies
slowly climbing toward 1.5 sigma. Lower levels will be slower to
warm given light winds / weak mixing through the length of period -
not to mention seasonally low sun angle. Also evident in temp
forecast - is residual 2-5" snow pack from southeast IND into west-
central OH which will be slow to erode given factors mentioned
above. While ECMWF ensemble suggests it may take 4-5 days to erode
this snow pack, it may be a little faster than that suggests.
However, snow pack will impact temperatures across the northern CWA
at least until Monday/Tuesday, keeping them in the 40s. As clouds
thicken and front approaches on Thursday, kept max temps in check
due to expected lack of sunshine - but still well above normal.
.AVIATION /02Z FRIDAY THROUGH TUESDAY/...
VFR conditions are expected through the entire period, although
there continues to be a hint of some very thin low level
moisture developing into northern portions of the region later
tonight into Friday morning. This could manifest itself in a FEW
clouds below 500ft or possibly some brief VSBY reductions, but
confidence is not overly high on either of these conditions
being met at even the northern sites. Did try to scale back a
bit in ongoing fcst but did not feel confident enough to yank
altogether. As such, have included a TEMPO group for KDAY, KCMH,
and KLCK for a few clouds in the 300-500ft range in the several
hour period around sunrise with some brief minor VSBY
reductions. However, this may even be overplayed a bit but
trends will undoubtedly be watched as we progress through the
Otherwise, light and variable winds with a gradual increase in
cirrus will be the main story for the period. Could very well
see the same low level moisture develop again Saturday morning
but will omit from the outlook for now as confidence remains low
in such an occurrence.
OUTLOOK...No significant hazards are expected at this time.
Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
846 PM CST Thu Dec 19 2019
845 PM CST
Only minor updates this evening were to nudge low temperatures up
ever so slightly and to refine the near-term patchy fog wording in
the gridded forecast.
The surface gradient continues to slowly relax this evening with a
col/saddle point noted in the surface flow just north of Waukegan
at this hour where a weak/diffuse backdoor boundary has slipped
southward down the lake. To the southwest of this boundary,
southwest to west-southwesterly flow in the low-levels is
promoting some modest warm advection ahead of a shortwave pivoting
towards western Minnesota. This slightly warmer airmass will
continue to inch across our region tonight which could result in
the development of some fog through the rest of the evening hours.
Already seeing some 5-6 mile reports at this hour, although the
combination of multi-degree F dewpoint depressions, increasing
high cirrus, and a very thin near-surface hydrolapse coupled with
a limited snowpack don`t seem to portend a widespread dense fog
potential. That said, the inherited area-wide patchy fog wording
tonight seems reasonable, with perhaps a corridor more favored for
some isolated pockets of lower visibilities north and east of a
Rockford to Evanston line where lowest-100 mb winds will be
lightest and the near-surface hydrolapse will be greatest. Updated
products have been transmitted.
223 PM CST
Only forecast concern is the potential for fog tonight into early
Friday. Temperatures have climbed back above freezing after a mid
week cold spell.
High clouds continue to spread out ahead of an upper trough back
across the Rockies and up atop an upper level ridge across the
plains into our area. The ridge axis will shift overhead tonight
which may keep the eastward extent of the clouds at bay. The ridge
will then spread to the eastern Great Lakes Friday. At the
surface, high pressure will remain parked across the eastern USA,
with a weaker low out ahead of the aforementioned upper trough.
The surface pressure gradient will temporarily weaken tonight as
the upper ridge passes overhead and with the surface high to the
east, southwest flow will usher in a bit more lower level moisture
and relatively warmer air over our cool and still partly snow
covered air mass. While this is a generally favorable pattern for
fog/low cloud development, the moisture advection is not all that
strong in this setup nor is the snowpack very thick or widespread.
Guidance remains mixed as to the degree of lower clouds and fog,
including the HRRR and the RAP which are not all that excited, but
there could be some localized pockets of thicker fog. The high
cloud cover may also act to insolate areas as well and mitigate
this too. Thus we continue with a light fog mention for the time
Certainly if clouds are not too extensive we would add at least
few degrees to highs on Friday. The low level inversion is stout
on Friday but shallow, thus we should be able to break anything up
that may form. Expect high clouds will thicken a bit more as the
upper trough approaches Friday afternoon.
245 PM CST
Friday Night through Thursday...
An extended period of mild and dry weather is expected to continue
through Christmas as broad, slowly progressive upper level ridging
moves across the central CONUS. Latest guidance continues to support
the warmest period to be Sunday through Wednesday. While the latest
model runs have backed off a little on max temps through this
period, with highs in the middle to upper 40s. However, the
GFS/ECMWF continue to indicate 925 mb temps of 10-12C for Monday,
which, climatologically, would support temperatures in the low 50s.
So, there is still the potential to see Max T on Monday in the low
50s should cloud cover be low enough to maximize insolation.
While mild weather will continue through mid-week, conditions should
be slightly cooler, with more extensive cloud cover and the passage
of a weak surface cold front whose associated upper system is of
Pacific origin. Unfortunately model solutions begin to diverge
significantly with the timing and strength of this system, so while
there is relatively high confidence in temperatures remaining above
normal, the timing differences introduce greater uncertainty to the
forecast by Wednesday, and especially beyond Wednesday, with regard
to pcpn chances. The GFS/ECMWF both are beginning to introduce pcpn
into the forecast for Tuesday and Wednesday, but the GFS is at least
18 hours faster than the ECMWF and the ECMWF develops a much
stronger mid-level shortwave moving across the region. So, have
kept PoPs at slight chance levels due to the uncertainty. The one
item of greater confidence is with thermal profiles, which should
remain high enough that any pcpn would be liquid, should there even
be any pcpn.
The message regarding chances for a white Christmas for nrn IL/nwrn
IN remains consistent. Chances for a white Christmas are about as
close to zero as possible without categorically stating that there
is a zero chance for a white Christmas. Statistically speaking,
with regard to forecasts of anything, weather or otherwise, there is
never an absolutely zero chance of something happening or not
For the 00Z TAFs...
Main concern through the TAF period remains the potential for
MVFR/IFR fog and/or stratus development by early Friday morning.
Confidence in this scenario remains low.
Winds continue to diminish this evening as the pressure gradient
weakens and higher pressure builds in from the east. Winds will be
predominately southerly through the period but there could be a
brief time where winds turn south southeasterly, although still
expect them to remain light.
Fog/stratus development is possible by early Friday morning
(6-10Z period). Light southwesterly flow in the low levels will
help advect at least some moisture into the area and with light
surface winds in place this is typically favorable for fog
development. However, an area of cirrus is moving in from the west
which could limit our surface cooling overnight somewhat which
could reduce our potential for dense fog. There continues to be
low confidence in how this will ultimately evolve. Have
maintained MVFR visibilities but will continue to monitor for the
potential for IFR vis/cigs. Our northern TAF sites look to have
the best potential for fog based on the latest guidance so have
included a mention of 3SM to hint at this trend.
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