Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 212145
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
345 PM MDT Sun May 21 2017

.SHORT TERM...(This evening through Monday)
Issued at 345 PM MDT Sun May 21 2017

A large upper low over the northern Great Plains and Great Lakes
region and an upper ridge nosing into the Pacific Northwest
through the period will keep northwesterly flow aloft. A shortwave
trough in this flow is spreading showers and thunderstorms over
the area this afternoon and into tonight. The system has slowed
slightly compared to previous forecasts, so have extended the
chance of showers and storms past midnight over areas south of
I70. Storms are still expected to remain weak, with light to at
times moderate rain and small hail possible. Low temperatures
tonight will remain below normal, with readings likely slightly
warmer over the southern half of the forecast area due to cloud
cover hanging on.

A stronger shortwave is expected to push down out of Montana
Monday on the nose of a 100kt jet. High temperatures will be
similar to today before a cold front pushes south across the area
late afternoon to early evening. Convection associated with this
shortwave and FROPA will likely produce stronger winds and
slightly larger hail compared to anything that is produced today.
North-northwesterly winds will be breezy in the late afternoon at
20 to 30 mph across most of the plains. Over the high country,
snow levels are expected to dip down to near 9000 feet tonight
then back up to 10000 feet Monday. Snow amounts due to showers will
stay around an inch at best.

.LONG TERM...(Monday night through Sunday)
Issued at 345 PM MDT Sun May 21 2017

Showers and storms accompanying the shortwave Monday night will
diminish overnight with drier air starting to work in from the
north. There will still be a little shallow instability over the
plains Tuesday. Not sure if it will be deep enough for rain, but
at least expect some convective clouds to redevelop. Temperatures
will be warmer than the last event, but still expecting some
snow accumulation in the higher mountains.

Ridge will be over Colorado on Wednesday, then it will be
flattened by a strong shortwave moving over the northern Rockies
Thursday. This will drag a cold front across the plains and
possibly give a little lift Thursday. For Friday and Saturday
there will cooler air with some moisture on the plains, with warm
and dry air aloft. Anything could happen these days depending on
the balance between the amount of cooling and moisture. Probably
marginal for convection, meaning late day storms. Possibly even
enough shear and instability for a few marginally severe storms,
but none of the ingredients are really very strong. Still
reasonable agreement on the mean trough axis moving to over or
east of Colorado during the weekend, which could bring deeper
moisture, another period of lift, and additional cooling. At this
point it still does not look like more than a day of enhanced
convection. There are hints of a secondary trough following on
Sunday or Monday which could do the same thing. Chance PoPs for
mainly diurnal convection and cooler temperatures for the weekend
look good.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 345 PM MDT Sun May 21 2017

With VFR conditions prevailing, the chance of weak showers and
thunderstorms will push over the terminals after 23Z today and
last through nearly 06z as an upper level disturbance pushes
overhead. Brief visibility restriction to 5SM, gusty winds to 30
knots, and ceilings down to 5000 feet or so possible with storms.
Otherwise ceilings 18Z-06Z should stay above 6000 feet, then clear
out. Wind forecast continues to be difficult before the expected
northerly push within the next couple of hours. Main push still
expected around 00z, with decreasing speeds after and a return to
a westerly enhanced drainage wind tonight. Another stronger
weather disturbance Monday will bring strong north winds and
stronger storms to the airports, after 21z.

&&

.HYDROLOGY...
Issued at 345 PM MDT Sun May 21 2017

After coordination with the Morgan County Emergency Manager, all
advisories have been cancelled due to the South Platte receding
over the western half of the county and the expected crest of the
river to not reach action stage at points downriver.

&&

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

$$

SHORT TERM...Kriederman
LONG TERM...Gimmestad
AVIATION...Kriederman
HYDROLOGY...Kriederman



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