Area Forecast Discussion
Issued by NWS Twin Cities, MN
FXUS63 KMPX 192058
Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
358 PM CDT TUE JUL 19 2016
.SHORT TERM...(This evening through Wednesday)
Issued at 350 PM CDT Tue Jul 19 2016
Confidence has increased in the heat wave for the middle of the
week, while today`s convection across Iowa has decreased the
confidence in the placement of heavy rain and potential for flash
flooding. The atmosphere is still primed to produce heavy rainfall,
but the location is quite uncertain. At this point the best
potential is across southern MN and northeast IA.
Early afternoon water vapor imagery showed a complex of
thunderstorms across southern IA that had persisted most of the day.
This produced severe weather and flash flooding across the state,
but also impeded the northeastward progression of the warm front.
Based off the 19.20 SPC mesopage, the H925, H850 thermo gradient was
lined up north/south across the MN/Dakota border. Later tonight the
RAP13 shows the H925/850 winds taking on a southwesterly direction
which should drive the Theta_E gradient eastward toward the MN/WI
border. This would be the focus for convection, heavy rain, and
Yesterday models indicated a shortwave forecast to move across the
region in concert with the warm front tonight. However this feature
is not easily discernible in afternoon water vapor imagery...so not
confident in a focused area of convection. The HiRes models show the
storms over Iowa waning, with some evening development across North
Dakota, but these storms are forecast to weaken by the time they
reach west central MN. Meanwhile, a third area of convection on
the tail end of the 19.15 HopWRF shows storms firing across
southern MN into northeast IA. Trended toward this solution since
in makes the most conceptual sense given the aforementioned
placement of the warm front at this time. The atmosphere is primed
to produce heavy rainfall with high freezing levels and elevated
PWATS together with slow storm motion and the potential for back
building. Despite the fact that it is quite possible that most
locations do not receive rainfall in the flash flood watch area,
continued with the watch since the potential remains.
.LONG TERM...(Wednesday night through Tuesday)
Issued at 350 PM CDT Tue Jul 19 2016
There remains no doubt that the heat indices Thursday afternoon will
be similar/comparable that occurred during the first week of July
2012. Based on pooling dew points along a weak front sagging
southward and ample sunshine, heat indices will rise into the 105
to 110 degree range Thursday afternoon. I would not be surprised
to see heat indices around the Twin Cities rising to 115 degrees
due to the heat island effect. Another interesting note is that
the atmosphere will be very unstable or almost explosive Thursday
afternoon/evening, but a unbelievable cap will keep thunderstorms
from developing along this front. If there is any weakening of
this cap due to subtle disturbances, chances of severe weather
will increase substantially. This front will not cool down the
atmosphere too much, however, it will decrease the dew points and
heat indices below 105 degrees Friday. Depending upon where this
front holds up, heat indices could approach 100 to 105 degrees
along the Iowa/Minnesota border Friday afternoon.
Although dew points will drop as this weak front moves through the
Upper Midwest Thursday afternoon through Friday, return flow will
develop Friday night, leading to more heat and humidity Saturday.
Models have continued to flatten the broad ridge across the center
of the country to a more progressive fast west to east flow over
the northern tier of the U.S. by this weekend. This is do to a
very significant short wave moving east-northeast across the
Pacific Northwest, and into the Northern Rockies/Northern Plains
by Saturday. As moisture increases once again as the flow becomes
southerly, chances of precipitation will increase late Friday
through the first part of the weekend. Timing remains questionable
as the GFS is much faster than the GEM/EC, but has slowed down
over the past few runs. Moisture depth will again be very
anomalous for even late July with precipitable water values rising
to 1.75" to over 2.0". Therefore, the potential of heavy rainfall
and flash flooding will develop as a series of short waves ride
this fast west to east flow aloft. Models have a significant
amount of QPF Friday night and Saturday morning as the first
strong short wave moves through the Upper Midwest. All severe
weather parameters are possible as this short wave passes through
due to strong shear aloft and high cape values.
Past this weekend, the flow remains west to west-northwest, with
even a possibility of a cutoff low developing next week across
the Upper Midwest. This basically means the heat will not be as
oppressive and the chances of precipitation increase slightly.
.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1258 PM CDT Tue Jul 19 2016
Upper level ridging is building north and east and will influence
the convective forecast over western areas of the forecast area.
Tricky forecast in terms of convective initiation tonight. Bottom
line is any convection north and west of KMSP will be very
isolated hence nothing in the terminals. Depending on what hi-res
solution we view there are various outcomes, but most favor very
isolated or no convection at all. The 12Z GFS and is on board with
convection south and east of KMSP and the 15z HRRRx supports that
through the early evening. It appears H700 temperatures are just
a tad to warm to have organized convection such as a MCS that was
previously discussed. Not overly comfortable making such a
change, but see no good reason continue chance TSRA. H700
temperatures are cooler to the south east and will keep a mention
as in PROB30 in the TAFs. We certainly have plenty of moisture
advecting northward but at this time it appears the main
convection will be located in Iowa. At any rate the forecast for
the next 12 hours bears watching closely for any subtle changes to
the atmosphere that would encourage organized convection. If
condition did develop expect heavy rain in the vicinity of TRWs.
Once we get past 15z Wednesday expect a fair amount of SKC with a
few puffs of cirrus noted and hot and humid.
KMSP...May see a few gusts this afternoon in the upper teens range
with a few cumulus. At this point in time I am doubtful we will
see any convection at the MSP terminal, but will keep a PROB30
for a few hours...just before daybreak. Otherwise SKC most of the
/OUTLOOK FOR KMSP/
Thu...VFR. Wind SSW at 5-10kts.
Fri...VFR. Chc -TSRA late. Wind SE 5 kt.
Sat...VFR. Chc -TSRA late. Wind S-SE 5-10 kt.
WI...Flash Flood Watch from 7 PM CDT this evening through Wednesday
morning for WIZ023-024-026.
Excessive Heat Watch from Thursday morning through Friday
evening for WIZ014>016-023>028.
MN...Excessive Heat Warning from 1 PM Wednesday to 7 PM CDT Friday
Flash Flood Watch from 7 PM CDT this evening through Wednesday
morning for MNZ051>053-059>063-066>070-075>078-083>085-092-
Excessive Heat Watch from Wednesday afternoon through Friday
evening for MNZ041>045-050-052-053-078-085-093.