Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 08/08/19

Area Forecast Discussion

National Weather Service Denver/Boulder CO

 820 PM MDT Wed Aug 7 2019

.UPDATE...
Issued at 820 PM MDT Wed Aug 7 2019

Forecast is evolving pretty much as expected this evening with the
strongest storms organizing over the eastern plains, roughly from
eastern Adams County through Lincoln county, and moving eastward.
The stronger storms are still producing heavy rain and some hail,
Storms may still produce heavy rain through midnight as they move
eastward into the higher Td (lower 60s) air and increasing low
level jet. Meanwhile, weaker convection in the Denver area is
already dissipating. We still expect a few showers and isolated
storms, however, as the weak short wave rotates eastward out of
Utah and across southern Wyoming/northern Colorado overnight.

Thursday is shaping up to be a wetter day (afternoon) with
heavier rainfall from storms as Precipitable Water (PW) values
increase. This is also handled well in the current forecast.

&&

.SHORT TERM...(This evening through Thursday)
Issued at 337 PM MDT Wed Aug 7 2019

Increasing moisture under an upper ridge is moving into Colorado
visible on satellite imagery as well as precipitable water values
up from yesterday. According to the latest ACARS soundings and
the current convective activity, a cap still resides from this
morning`s cool push into the plains. Plenty of showers and storms
firing over the mountains, but they diminish as they head out over
the plains. Clearer skies over the plains should still allow for
a couple more degrees of heating which should weaken the cap as
well as stronger storms to the north and south of our forecast
area should send out some outflow for additional forcing.
Therefore still keeping the scattered to numerous showers and
thunderstorms later this afternoon pushing east over the plains
through the rest of the evening. Weak CAPE values over the urban
corridor and mountains increases east over the plains, though
some models keeping CIN over the far northeastern corner. Best
chance for stronger storms looks to be keying into Washington
County. A shortwave noted over Utah will round the ridge and push
into the forecast area this evening, extending the chance of
showers and storms across the area through midnight, with the
chance continuing over the eastern plains through sunrise.

An MCS will likely develop east of our area late tonight into
tomorrow morning, which should end up sending back moist outflow
into the area, helped along by high surface pressure over the
northern plains. This will increase upslope flow, with an already
moist airmass in place. Showers may be ongoing over the mountains
overnight and into Thursday morning with just a slight bit of sun
needed to get the plains going again. PW values should increase
slightly over today over the mountains and urban corridor, with
higher values near 1.4 over the northeastern plains. Slightly
warmer temperatures east of the Palmer Divide combined with
upslope flow and influx of moisture will increase CAPE values into
the 1500-2200 J/kg range in this area. Therefore, storms that form
or move into that area will have the potential for heavy rain,
hence the slight chance of excessive rainfall forecast from WPC.
Temperatures tomorrow will be about 2 to 8 degrees F cooler than
today`s readings, and should drop several degrees below
climatologically normal for the date.

.LONG TERM...(Thursday night through Wednesday)
Issued at 337 PM MDT Wed Aug 7 2019

Medium range models continue showing a prolonged period with the
potential for heavy rainfall from afternoon shower activity each
day. Early in the forecast period, the upper level ridge will be
centered over eastern Colorado which will allow for a strong surge
of sub-tropical moisture to move over the state. Precipitable
water values should be high, and winds aloft will be weak, leading
to the formation of slow moving thunderstorms that are laden with
moisture. shower activity Thursday evening should be left over
from afternoon initiation over the mountains that moves out onto
the plains. A similar setup should be in place Friday afternoon,
but with a little more southwesterly flow aloft. Heavy rainfall
will again be a threat, from the foothills out onto the plains.

Over the weekend, the moist southwesterly flow aloft will
continue as the upper ridge axis shifts eastward over the Central
Plains. More afternoons with the threat of heavy rain will exist.
The GFS shows an embedded short wave trough rotating over the
state Sunday afternoon to further help the organization of the
developing convection.

For next week, the flow aloft across Colorado is expected to be
more westerly, but models also show that sub-tropical moisture
will continue feeding into the region from the southwest. Until we
see stronger upper level ridging develop over Arizona and Nevada,
it seems that the monsoonal flow of moisture will continue over
the desert southwest. Until then, the cycle of afternoon and
evening thunderstorms over northern Colorado will continue.

Temperatures through this period should be around normal for
early to mid August, with high temperatures on the plains reaching
the mid 80s to lower 90s before afternoon shower activity develops
and brings some cooling. As the afternoons of heavy rain
accumulate, then the threat for flash flooding will begin to
increase. No highlights at this point.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 820 PM MDT Wed Aug 7 2019

Shower and storm chances are slowly decreasing this evening, but
an isolated storm still possible til about 06Z. East/northeast
winds at KDEN and KAPA will transition to normal south/southwest
winds slowly through 06Z-08Z. There may be a weak frontal
passage/surge 12Z-15Z Thursday which will bring a chance of
stratus development with IFR/MVFR ceilings possible, otherwise
VFR conditions should prevail through about 21Z Thursday.
Thunderstorm chances will then increase considerably by 21Z
Thursday under a more moist and unstable airmass, with VIS and CIG
restrictions and gusty/variable winds to 35-40 knots under the
stronger storms.

&&

.HYDROLOGY...
Issued at 820 PM MDT Wed Aug 7 2019

Storms with locally heavy rain may produce minor flooding in rural
areas on the eastern plains until about midnight.

On Thursday, due to the influx of moisture into the area, as well
as instability, storms will be able to produce rainfall rates of
1 to 2 inches per hour, especially near and east of the Palmer
Divide. This could create localized flooding issues in the
aforementioned area.

&&

.BOU WATCHES/WARNINGS/ADVISORIES...
None.
&&

$$

UPDATE...Barjenbruch
SHORT TERM...Kriederman
LONG TERM...Dankers
AVIATION...Barjenbruch
HYDROLOGY...Kriederman/Barjenbruch



Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

 1026 PM CDT Wed Aug 7 2019

.UPDATE...
841 PM CDT

Continue to monitor thunderstorm potential tonight, mainly after
midnight, across portions of northern IL. Thunderstorms of
interest are currently across central WI, developing along
surface trough/front. This trough/front is expected to sag south
through the overnight hours, with the potential for thunderstorms
to increase across the CWA. However, the large scale ascent, which
the current development across WI is feeling at this time, will
stay to the north. So, do anticipate a diminishing trend with any
southward moving storms as well as coverage to lower with mainly
dry conditions expected for most areas. That being said, with the
front swinging through, can`t completely rule out isolated
development tonight along it and have maintained mention of
thunder in the forecast. Expect lower level instability to
decrease tonight, and if anything does develop, don`t anticipate
any strong/severe storms.

Rodriguez

&&

.SHORT TERM...
227 PM CDT

Through Tonight...

The forecast and reasoning for tonight has changed very little.
There remains potential for some scattered, semi-organized storms
moving into northern Illinois, more so far northeast Illinois,
and far northwest Indiana after 10 p.m. tonight. A few of these
storms if organized could produce 40+ mph gusts.

A normal August afternoon with temperatures in the mid 80s area-
wide and a scattering of cumulus, along with a lake breeze
struggling to advance inland. There has been some clumping of
cumulus along the lake breeze convergence but aircraft sounding
data indicate any towering updrafts would be narrow with dry air
entrainment and that has been the case. This lake breeze should
retreat late afternoon with downtown Chicago likely to jump from
the upper 70s to mid 80s sometime between 5-7 p.m.

A 1004 mb surface low just north of Green Bay is steering a cold
front southeastward across northwest Wisconsin and resulting in
scattered thunderstorms. Convective sustenance parameters are in
place for the storms to uptick through early evening into central
and eastern Wisconsin with shear profiles favoring some
organization into an MCS, which convective allowing models
generally support. This would be most favored to be on a
trajectory to move over central Lake Michigan and into Michigan
and not our area. Further west of this -- more upstream of the
forecast area -- more orthogonal shear orientation with respect to
the initializing boundary would support scattered to loosely
organized convection. This should track southeast with any
convective segments that surge southeast most capable of the
stronger winds, and that is possible as far south into the
northern CWA given the stronger mid to upper level winds. But the
storms will be fighting waning instability, and the instability
already is marginal over northern Illinois and far northwest
Indiana. Feel that the best chance of storms (around 50 percent)
is in far northern suburbs of Chicago (Lake Co area).

Any convection that does make it into the area should be weakening
and/or moving out of northwest Indiana by 3-4 a.m. The cold front
should move through overnight/pre-dawn time period, also in a
weakening phase.

MTF

&&

.LONG TERM...
218 PM CDT

Thursday through Wednesday...

For much of the long term period the main feature impacting the
weather across northern Illinois and northwestern Indiana will be a
large, sprawling high pressure system dropping out of Canada
Thursday night and then spreading across the Middle/Upper
Mississippi Valley Friday through Saturday.  Temperatures will be
near seasonal normals, but given the continental polar air mass
associated with the surface high, dewpoints should be lower than
normal for for mid-August. While there is some concern that a few
scattered shra/tsra could develop along the cold front expected to
push through the region during the Short Term forecast period at the
most favorably diurnal time, Thursday afternoon, the higher chances
for more widespread pcpn will come later in the period.  The next
chance for pcpn should be Saturday night as the longer range
guidance continues to advertise a weak shortwave rippling through
the northwest flow aloft that will be entrenched across the area,
between upper ridging over the wrn CONUS and broad long wave
troughing over the ern CONUS. The upper ridge is expected to build
east through the weekend and into early next week.  This will set up
more zonal flow aloft across the Midwest.  the longer range guidance
indicates a series of waves rippling through the strengthening wly
flow aloft, but confidence in timing of individual waves will
quickly diminish as the upper flow becomes more zonal.  So, while
the official forecast will indicate periodic chances for showers and
thunderstorms from Sunday afternoon into Tuesday night, confidence
in exact timing of any individual wave is low and there will likely
be periods of dry weather intermixed with the chances for pcpn.
Also, lake breeze development will likely be an almost daily
occurrence, so daytime highs along the lakefront will be a little
lower than inland highs.

&&

.AVIATION...
For the 03Z amendment...

Elected to add a period of showers and temporary wind shift
centered on the 06z timeframe to account for the activity working
south across Wisconsin. Still have low confidence that this line
will hold together as it moves into a less favorable environment,
thus the choice to omit thunder at this time. Later trends may
require a different approach, either to add thunder or remove the
mention of showers altogether. A potential wind shift to the
northwest would appear to be short lived and should not have
significant operational impacts, especially at this time of night.

For the 00Z TAFs...

Pockets of convection well north of the area across Wisconsin
could work their way into the local terminals overnight with the
approach of an associated cold front but trends are not appearing
favorable for this activity to hold together. If it does it would
most likely be overnight when traffic demands are lowest, and then
departing the area by the time traffic picks up in the morning.
Otherwise expect VFR with persistent west to northwest winds,
diminishing overnight and likely gusting again tomorrow to around
20 kts.

Lenning

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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