Area Forecast Discussion
Issued by NWS Denver/Boulder, CO

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FXUS65 KBOU 240155
AFDBOU

Area Forecast Discussion
National Weather Service Denver/Boulder CO
755 PM MDT Tue May 23 2017

.UPDATE...
Issued at 755 PM MDT Tue May 23 2017

No big changes needed to the ongoing forecast. Winds are
decreasing with skies clearing, except for high level clouds
moving down from the north.

&&

.SHORT TERM...(This evening through Wednesday)
Issued at 233 PM MDT Tue May 23 2017

Light rain showers have been occurring this afternoon as expected
generally east of the Denver metro area. The showers have
developed in response to mesoscale lift from the left exit region
of a 100kt northerly jet streak. GOES-16 and high res model
guidance continue to show the jet streak propagating east, placing
the core of jet across the eastern half of Colorado later this
afternoon and reducing the mesoscale lift across our area. The mid
and upper level ridge axis is also approaching our area this
evening and will be overhead by Wednesday midday, which generates
synoptic scale subsidence. All that said, rain showers will be few
and far between the remainder of today, confined to far eastern
Colorado. Any rain that manages to reach the ground will be light
given surface dewpoints in the mid 30s across the Plains. Closer
to the ridge axis and without any jet support, convection in the
mountains today should be suppressed. In general, any rain shower
activity across NE Plains of Colorado should be over before
sunset and PoPs have been removed after 6 PM.

Tonight under the influence of the approaching ridge, skies will
clear and the winds at the surface will relax with a weak
pressure gradient in place across our area. Temperatures will cool
nicely tonight into the upper 30s away from the urban heat islands
and away from the foothills where drainage winds should keep
temperatures in the low 40s. At this time it does not look cold
enough to warrant a frost advisory across the far eastern Plains
but in the coldest valley locations in Lincoln and across far NE
Colorado temperature/dewpoint could get close to a frost.

On Wednesday strong subsidence downstream of the ridge, weak
downsloping, and a thermal ridge at 700 advected across the area
all point to a major warm up. 700 mb temps warm 12C in 24 hours!
Should have little trouble reaching the upper 70s to low 80s
across the Plains and 60s most mountain locations Wednesday
afternoon. At this time instability across the Plains is limited
despite the warm surface temps, as surface dewpoints will remain
in the upper 30s to low 40s. Any weak convection that develops
across the mountains and drifts east could be capable of brief gusty
winds given the well mixed boundary layer and 40 degF T/Td
spreads, but no rainfall is expected our of the weak convection
areawide. Lee troughing with weak westerly flow throughout the
day will result in west winds most areas, but the pressure
gradient remains weak.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 233 PM MDT Tue May 23 2017

Wednesday night should be fairly quiet through 06z, then clouds
and showers will start to increase from the west as a subtropical
jet max approaches the four corners region in the morning and
advects eastward through the day. Weak mid level qg ascent will be
in place coupled with a weak cold front pushing in from the north
in the morning. This should result in some light pcpn over the
mountains by 12z Thursday, then isolated showers over the
northeast plains Thursday morning. The coverage will increase
across the cwa in the afternoon with a chance of thunder as well.
The best potential will be in and near the Palmer Divide Thursday
aftn with boundary layer CAPES 900-1000 J/kg by 21z. The forecast
sounding suggests an inverted-v profile, so strong outflow winds
with minimal rain showers for the Front Range. The next system
over southern ID Thursday night, will gradually move
east/southeast across northern CO on Friday through Saturday
morning. A persistent but relatively weak mid level qg ascent will
be over the region through this period, with a stronger cold
front pushing into Denver around 00z Saturday. The models show a
sharp increase low level moisture Friday afternoon with CAPES
close to 2000 j/kg, but it appears to be fairly capped as well. At
this time, it looks like a better chance of light to moderate
rainfall with some embedded thunderstorm. If severe thunderstorms
occur, then the best potential will be south and southeast of
Denver along the Palmer Divide Friday aftn. The flow aloft will
transition fm southwesterly to northwesterly by midday Saturday,
with moderate mid level subsidence behind the trough axis Saturday
aftn and evening. If this timing is correct, then the best chc of
showers on Saturday will be Saturday morning. For now will hold
on to a chance of showers and tstms through the day. Overall
temperatures will decrease Thursday through Saturday with more
cloud cover and the passage of a couple of cold fronts. Sunday
through Tuesday, the flow aloft will remain northwesterly with
building ridge over the western U.S. The best chc of showers at
this time will be over the higher terrain during the aftn and
early evening.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 755 PM MDT Tue May 23 2017

VFR conditions through at least Wednesday for all three
TAF sites. Northerly winds will shift to normal drainage out of
about 190deg after 06Z. Tomorrow expect west winds after 17Z at
the TAF sites but given a weak pressure gradient, speeds should
remain 10 kt or less throughout the day.


&&

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

$$

UPDATE...Kriederman
SHORT TERM...Schlatter
LONG TERM...Cooper
AVIATION...Kriederman



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