Area Forecast Discussion
Issued by NWS Twin Cities, MN

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FXUS63 KMPX 201047

Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
547 AM CDT Thu Jul 20 2017

.updated for the 12z aviation below...

.SHORT TERM...(Today and tonight)
Issued at 338 AM CDT Thu Jul 20 2017

The short term concern is timing next bout of convection later

The heavy rain producing convection has exited the cwa for the most
part. We did receive 2 to 5 inches of rain from the south metro to
along the Mississippi River to the southeast. Should be a mainly dry
day into this evening as the frontal boundary sags farther south
into Iowa and high pressure builds into the state. Some patchy fog
developed into central MN as the clouds cleared and this should burn
off quickly this morning. High temperatures are expected to warm
through the 80s.

Later tonight we see return flow/low level waa developing to the
southwest along with a possible short wave moving into southwest MN
from South Dakota. We will retain the chance PoPs for that
development later tonight. Clouds will thicken over the southwest as
well. Farther east, with light winds and the possibility of a mostly
clear sky much of the night, we could see some patchy fog develop
into west central WI later tonight as well.

.LONG TERM...(Friday through Wednesday)
Issued at 338 AM CDT Thu Jul 20 2017

In the big picture, we`ll see fairly zonal upper flow on Friday
transition to northwest flow by sunday with cooler and drier air
working into the region. However, an upper trough looks to dig
along the west coast during the first half of next week, leading
to the redevelopment of upper ridging over the center of the CONUS
by Tuesday/Wednesday. This looks to remain in place through the
remainder of the week. So, although we briefly see temperatures
dip below normal for Sunday/Monday things should quickly return to
at or above normal values for the remainder of the period.

In terms of the details, the main concern will be the possibility
for a few more rounds of showers and thunderstorms Friday into
Saturday, with locally heavy rainfall and some possibility of
severe weather again. The warm frontal boundary looks to get up
into the southern portion of the area on Friday as a weak surface
low slowly moves east from the central and northern Plains. Plenty
of instability and moisture will be present across the southern
half of the area, so the modest warm advection forecast should be
more than sufficient to get showers and thunderstorms going again
on Friday. The steering flow looks to be somewhat parallel to the
baroclinic zone and instability gradient, so some training of
storms seems possible, which would lead to a heavy rain threat,
particularly over the southern portion of the area. The surface
low will slowly push east Friday night and Saturday, working into
the lower Great Lakes by Saturday night. The guidance is
consistent in showing an inverted trough/occluded front north of
the surface low, and we may see additional showers and storms
develop near/ahead of that on Saturday, primarily impacting
locations near and east of I-35. The main cold front looks to drop
south across the area Saturday night into Sunday morning, with
high pressure building in for Sunday and Monday. We`ll get on the
backside of the surface high by Monday night and Tuesday, with
return flow setting up and a renewed chance of showers and
thunderstorms working in from west to east Tuesday and Tuesday
night. The guidance has some significant differences at that point
in time with respect to how far north the baroclinic zone and low
level moisture return, and don`t have particular confidence in any
specific solution at this point. The GFS would hang onto more in
the way of northwest flow into the second half of the week, while
the Canadian is more agressive in building the ridge over the
area. A compromise may be best at this point, which the ECMWF
somewhat suggests. Regardless, it appears we`ll be near the
periphery of the ridge and instability gradient, so chances for
showers and thunderstorms look to be a good bet into at least


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 545 AM CDT Thu Jul 20 2017

Convection has exited the area to the southeast. Patchy high
clouds over the northern areas and light winds have made
conditions favorable for fog at least early this morning at most
locations. Mentioned some fog combinations through 13z. Then
expect vfr through at least 06z Fri as high pressure drops south.
Clouds expected to increase again to the southwest later tonight
and may usher in some -shra/-tsra toward KRWF before 12z.
Mentioned vcsh for now late. The rest of the area should remain
dry with perhaps some fog into west central WI late if high clouds
arent thick enough. Light north winds for the most part today
becoming light southeast late.

Looks like some BR early...burning off by 13z. Then vfr through
period with high clouds dropping to mid clouds ;ate in taf period.
Mentioned vcsh after 15z Fri for the next chance of -shra moving

Fri night...Mainly VFR. -SHRA/TSRA likely. Winds SE 5-10 kts.
Sat...Mainly VFR. Chc MVFR in -SHRA/TSRA. Winds lgt and vrbl.
Sun...VFR. Wind NW 10 kts.




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