Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 230157
AFDMPX

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
857 PM CDT FRI JUL 22 2016

.UPDATE...
Issued at 851 PM CDT Fri Jul 22 2016

Updated to include 00z aviation discussion below.

&&

.SHORT TERM...(This evening through Saturday)
Issued at 335 PM CDT Fri Jul 22 2016

Only concern this evening is a few isolated thunderstorms near the
Iowa/Minnesota border as the front (dry line...) has stalled.
There is also some thunderstorms in northeast South Dakota, but
these should follow the moisture gradient to the south-southeast
across southeast South Dakota and into far southwest Minnesota.
However, there is some uncertainty on when the stalled front moves
back northward overnight. So, a few thunderstorms may develop from
outflow boundaries from the storms in eastern South Dakota into
west central, and southwest Minnesota before midnight.

The main increase in precipitation coverage will occur late
tonight as the front finally lifts northward and the low level
jet increases along the strengthening theta-e gradient. This
should begin in southwest-south central Minnesota and expand
northward during the pre dawn hours. Although, typically the low
level jet lessens in the morning, which leads to an overall
decrease in thunderstorm activity, this may not be the case.
Another scenario could be the continued development of convection
forming northward during the morning as the theta-e gradient
tightens. In addition, this highly induced theta-e gradient/warm
front scenario will likely be the focus for more convection
during the afternoon in central Minnesota. This is also where the
best potential of back-building thunderstorms, and high rainfall
rates will increase the flash flood threat. Therefore, a flash
flood watch has been issued for this reason.

Models have indicated that 6 hour precipitation totals could
reach 1 to 2 inches in the watch area between 18z Saturday - 06z
Sunday. If you combine the added affects of isolated rainfall
rates of 1-2 inches per hour, you could estimate totals of 4-6
inches in the watch area before it tapers off sometime Saturday
night. Later forecast shifts can redefine the area of concern once
thunderstorms develop overnight.

.LONG TERM...(Saturday night through Friday)
Issued at 315 PM CDT Fri Jul 22 2016

The long term trends continue to show a possible heavy rain event
across the eastern cwa into Saturday night with the passage of
the upper trough and cold front. Several cams suggesting two
convective events...ongoing over the eastern cwa by 00z Sunday and
any possible redevelopment over the west during the evening with
the passage of the front. SREF 3hr calibrated severe thunderstorm
forecast shows a good likelihood of at least scattered severe
with the system. We like the HRRR exp and NMM-wrf as far as
evolution of the event. Heavy rain will remain a factor as
well...as the PW`s increase to near 2 inches ahead of the front.

The system exits the area Sunday morning with a couple of dry
days expected. The next system arrives toward Tuesday and the GFS
and ECMWF show the chance of showers and storms continue through
the end of the week. Model discrepancy/timing differences of
embedded short waves is strong enough to leave the more broad
brush approach to the the precipitation forecast at this time.
Overall...temperatures are expected to trend cooler by late in
the period as the middle conus ridge does appear to break down
more bringing in cooler Canadian air.

&&

.AVIATION...(For the 00Z TAFS through 00Z Saturday evening)
Issued at 851 PM CDT Fri Jul 22 2016

VFR conditions expected through the overnight hours. Tricky
forecast then starts from 12z onward due to timing of convection.
Short-term models generally indicate two rounds of convection: one
in the late morning to early afternoon hours, another in the early
to mid evening hours. Have tried to time these using TEMPO groups
without plastering VCTS or VCSH throughout the TAFs but confidence
remains not great due to only modest agreement at best among the
models for not only the timing but even the potential for more
than one round of convection. Should the early round take up most
of the energy and prevent additional insolation, then the
potential second round may not develop as much, or even at all,
for some terminals. So, safe to say, additional TAF refinements
will be forthcoming in the upcoming issuances.

KMSP...Not much more can be said for MSP at this point as
confidence is fairly high that thunderstorms will occur at the
field but it is the timing that is in question. Have, at this
point, highlighted late morning and then early evening but this
timing can certainly change. If anything, the later occurrence may
be earlier than currently advertised but will need to see how
additional model runs play out the scenarios.

/OUTLOOK FOR KMSP/
Sun...VFR. Winds NW 10-15 kt.
Mon...VFR. Winds NW 5-10 kt.
Tue...VFR. Thunderstorms Possible. Light & Variable Wind.

&&

.MPX Watches/Warnings/Advisories...
WI...Flash Flood Watch from Saturday afternoon through Sunday morning
     for WIZ014>016.

MN...Flash Flood Watch from Saturday afternoon through Sunday morning
     for MNZ043>045-050-052-053.

&&

$$

UPDATE...JPC
SHORT TERM...JLT
LONG TERM...DWE
AVIATION...JPC



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