Today is a tricky day to forecast for the Northern Wasatch. With the NWS discussion boldly stating how exceptional the precipitation events could be for the day in southern and eastern Utah, it was hard to corroborate the comments when looking at the water vapor images for the morning. The moisture from tropical storm Ivo was clearly visible and pushing north, but a drier than expected air mass was positioned to the west. So... What to do?
We spent the first 2 hours going over the comp details at HQ, which gave ample time to review how large the cumulus were getting. By noon our meeting was over and a thick cirrus layer was overhead, with a clear line stretching across the sky like a wet blanket--it was suppressing the convection and no thunderstorms (TS) developed in the region.
We headed to launch. Waffled a bit, listened to Dave Dixon present a nice speech on key elements to overcoming the mental game of XC flying... By 2:30 Ken decided to cancel the day. Most pilots opted to fly down to get a sense of the area in smooth, buoyant air. Some thermaled in weak lift for well over an hour. I was clearly not a day for any XC flying and given the wild card of potential embedded cumulus overhead, and more moisture on the way, it was the right call.
MODIS image taken at around Noon.
MODIS image taken around 3pm
So what about the forecast? I'll post the full discussion notes, because it sets the stage for the week quite well. From 5:50 AM.
Area forecast discussion
National Weather Service Salt Lake City Utah
550 am MDT sun Aug 25 2013
Synopsis...an exceptionally moist southerly flow will remain over
Utah through the early part of the week. Widespread precipitation is
expected with heavy rain possible across mainly the southern half of
the state through Monday afternoon.
&&
Discussion...a southerly flow pattern remains in place over Utah
this morning. The main features of note upstream of the area include
an upper trough deepening over the California coast and Tropical
Depression Ivo off the Baja California coast. Although the airmass has dried a
little across the west...increasing south flow will draw deep
tropical moisture from Ivo up into the area today through tomorrow.
Significant cloud cover from this moisture tap is already in place
across Arizona and is spreading into Utah. Precipitation is
currently increasing across southeast Utah and will continue to
develop across much of southern Utah this morning into this
afternoon. The bulk of the precipitation can be expected across
southern...central and eastern Utah. Expect widespread locally heavy
rain with embedded thunderstorms in these areas through Monday. The
large amounts of rainfall expected will likely result in enhanced
runoff in many areas and lead to rises in rivers and streams as well
as dry washes and slot canyons. A Flood Watch remains in effect for
this afternoon through Monday.
Although the deep moisture will not quite make it into the
northwestern third of the forecast area...expect that isolated to
scattered convection will still develop today. Precipitable water
values remain around 0.8 inches there...and given the clear skies
this morning...there will be plenty of surface heating to help
destabilize the airmass by this afternoon. Additionally...while
coverage may be somewhat sparse...these storms could have some
organization to them given a strengthening jet across far northwest
Utah associated with the aforementioned trough lifting across the
northwestern states. Gusty winds will be the primary threat with a
small potential for hail as well.
As what remains of the core of the tropical depression weakens and
lifts across the area Monday night...the heavy rain potential will
decrease behind it. The airmass will dry out a bit...but since it is
already starting out extremely moist and because a southerly flow
will remain in place...plenty of moisture will linger across the
area to keep showers and thunderstorms active through late week.
However...with the absence of any significant large-scale
lift...will see a transition back to more typical monsoonal
convection rather than widespread heavy rain.
The onset of meteorological autumn will bring an easing of the
monsoonal pattern as the jet stream strays farther south and brings
middle-latitude troughs closer to Utah. Global models indicate one such
trough will graze northern Utah over the weekend...potentially
sending a weak cold front into the area and drying the airmass for
what looks to be an extended period of time.
OK. So interesting.And what about the forecast models, like XC Skies? Sky cloud cover did well for noon.
And looking at the evolution of moisture in terms of RH over Provo from 6am to 6pm looked like this:
And so what of the diminished lift on the shaded side of this feature? The NAM model shows it well. With reduced surface heating from cloud cover, but increased moisture destabilizing the region, these scenes show what's going on clearly:
with reduced surface heat flux due to cloud cover, the upward momentum is diminished. This means the top of usable lift is much lower than the full depth of the boundary layer. The reduced vertical velocities reflect this as well. All of these are coupled tightly and tell the same story.
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