Preliminary Analysis of IOP5 Storm

We're starting to get a look at the Doppler on Wheels (DOW) radar data from the Friday Night–Saturday Morning storm and it's quite remarkable.

First, here's a loop of the lake-effect precipitation.  The pac-man-like radar coverage extends from the southeastern Great Salt Lake (top) to just west of south of Point of the Mountain (bottom).  The red reflectivity maxima that don't move are the Wasatch Range.  The central Wasatch are between about 90 and 135 degrees in this image.  

Click for this to loop (big file)

A strong lake-effect band exists at ~1245 UTC (0645 MDT).

We can take vertical slices through the storm along the redish/blue lines above.  One cuts through the lake band, the other through the Broads Fork Twin Peaks.  In the resulting image below, the wedge on the right cuts vertically through the lake band, the other across the Salt Lake Valley and into the Broads Fork Twin, which is the diagonal radar reflectivity maximum (red) on the extreme left hand side.

What is most remarkable about this image is how shallow the precipitation is.  It barely extends to mountaintop level.  I always tell people that lake-effect storms can be quite shallow and that you can often see the sun through the clouds when in the mountains, but this is really incredible.

Event Winding Down

The lake-effect is beginning to "detatch" from the lake and weaken in intensity and coverage.

Below is some eyecandy from the DOW showing a vertical RHI scan through the lake band and embedded convective cores earlier in the event.

Great job by everyone on this one.  University of Utah Atmospheric Sciences students are the first to collect the data, the first to analyze and publish the data, and the first to ski deep powder in the Wasatch after an all nighter.  I'm not sure if the powder skiing is all that practical given our meager snowpack at present, but I'm sure they will make up for it in a few weeks.

Great Lake Band

Good things come to those who wait.  We now have a remarkable lake band event underway.

The Doppler on Wheels is ideally situated for looking at the evolution of this band over the Salt Lake Valley and, in addition, the orographic enhancement over the Wasatch range, which is apparent in the loop below (note the darker shades of green).  Wow.  The Wasatch Weather Weenies have hit a home run on this one!

Update: 711 AM MDT

Just spoke with the DOW team and they are killing it.  In addition to the lake band, the Wasatch are lit up and they are doing scan after scan over the highest peaks.  This is a great mixed mode event with both lake and orographic processes in play.

Lake Effect and Other Weird Stuff

Over the past hour we have seen a broad band of precipitation develop to the south of the Great Salt Lake with embedded convection (>35 dBZ) over the south arm of the lake.

The convection over the lake is likely lake enhanced, so I'm willing to call this a lake-effect event.  Why the broader precipitation shield extends so far from the lee of the lake is a bit of a mystery, but perhaps larger-scale processes are contributing since we still have the trailing portion of the frontal cloud and precipitation band over us.

Winds on Promontory Point are out of the northwest and there is a clear contrast between the flows to the east and west of the lake.  Along the Lakeside Mountains, the flow is out of the west, whereas it is more northwesterly in the Ogden area, indicating the development of overlake convergence.

The westerly flow over the lake-side mountains has intensified over the last couple of hours, which we see as a good sign for the lake-effect to continue and perhaps spread into the Salt Lake Valley.

Indeed we're hoping that things will shift to the east a bit as the Oquirrh Mountains block our view from the DOW of the Tooele Valley and western Great Salt Lake.

Finally, the relative humidity at Lakeside Mountain and Locomotive Springs to the north of the lake is 100% and 77% respectively.  It never hurts to have upstream moisture to play with.

Late Night Joint IOP Update

Precipitation continues but is now scattered and generally light across the Wasatch Front.  The latest radar loop suggests that a shallow layer of cold, northwesterly lies over the lake.  Radar echoes are still moving out of the southwest over much of the Salt Lake Valley, but out of the northwest over the lake.

There is one weak, wind-parallel band over the lake that make be lake induced.  We'll have to see if it continues.  We hope that we'll see bonafide lake-effect develop as we get into the early morning hours, the cold air deepens, and the land breezes get going.  Even if it doesn't, we'll have a good dataset from the polarimetric radar of a prolonged precipitation event that will be great for our students to play with.

We're in Business!

I love it when a band comes together!

It is now raining or snowing across much of the Salt Lake Valley, with the white stuff beginning to stick in the upper Avenues.

Thanks to the marathon efforts of our friends from the Center for Severe Weather Research (especially Ab), we picked up the Doppler on Wheels (DOW) in Tooele at about 8 PM and our team deployed immediately to our SR-111 observing site.  We now have data flowing in from this storm.  So psyched.

Looks like a few hours of examining the frontal band and orographic precipitation processes are guaranteed.  I'm keeping my fingers crossed that we'll see the lake get going thereafter.

Storm Chasing Plans

A brigade of students is heading out over the next several hours to examine tonight's storm.  The effort will serve as the first joint intensive observing period (IOP) of the two major field projects we have underway this fall.  They are the Storm Chasing Utah Style Study (SCHUSS), which involves educational use of the famed Doppler on Wheels radar, and the Sounding Observations of Lake-effect Precipitation Experiment (SOLPEX), a National Science Foundation sponsored project designed to improve our understanding of lake-effect storms and their interactions with complex terrain.

We have mobile upper-air sounding teams deploying to the Kelton "ghost town" to observe the atmosphere upstream of the Great Salt Lake and to west of Syracuse to examine mountain and lake-induced flows near the east shoreline.  We also have a car-mounted mobile weather station heading out and the Doppler on Wheels, which we hope will arrive for use by 10 PM from its exhibition in San Francisco.  It's currently somewhere in central Nevada!  Curses to Cal-Trans for closing Donner Pass last night!!!

Being an occasional golfer, I'll describe this as a high risk–reward event.  The storm is complex, the models suggest it has low predictability, and lake-effect is always a crap shoot.  The models have struggled with it all week.  At the moment, the surface front is pushing into the Wasatch Front.

There is no organized precipitation along the surface front, but scattered precipitation is trying to develop over northern Utah.  The prefrontal airmass is quite dry, however.  At KSLC, the relative humidity at 1553 MDT (2153 UTC) was 16% with a temperature of 61F and dewpoint of 15F.  Things aren't much better behind the front.  At roughly the same time at milepost 29 on I-80 the relative humidity was 45% with a temperature of 46F and dewpoint of 26F.  As a result, we presently have "virga all quads."

Prefrontal conditions at 1520 MDT (top, looking west from
Ensign Peak Ridge) and 1530 (bottom, looking south
from above City Creek Canyon)

Strong dynamics and moister air from the north and west are needed to juice this thing up.  The models believe this will happen, producing a broad band of precipitation initially and then conditions that could produce lake-effect tonight.  We are keeping our fingers cross that Mother Nature brings the goods and that the DOW arrives safely.

Update at 640PM
 The DOW has just crossed the border!  If all goes well, we'll be positioned and operating by 9:30!

Planning for Storm Chasing

Photo: Josh Wurman, Center for Severe Weather Research

University of Utah students who are interested in learning more about winter storms and getting hands on experience with polarimetric radar are invited to attend a meeting at 2PM today (Friday 16 Sep) in 711 WBB.  As announced earlier this month, the famed Doppler on Wheels research radar will be deployed to the University of Utah from about 21 Oct – 21 Nov and available for student-directed research projects.  At today's meeting, we will be identifying some key phenomena and research issues to examine, and forming teams for developing deployment strategies, including siting and scanning sequences.

SOLPEX IOP5: Winding Down

The SOLPEX IOP5 lake-effect event is winding down quickly now as these events typically do with surface heating.  Radar echo coverage has declined rapidly over the past hour and is now quite limited.

Concurrently, the cloud coverage over the SE corner of the lake is beginning to break up.

The flow over the northern Wasatch Front is beginning to show signs of shifting to westerly.  There are a couple of stations in that region that now have a light west wind, including West Point.

These changes are subtle, but show the dynamical forcing for the event is in decay.  We're going to do a launch at 11 am to make sure we capture the full life-cycle of the event and then bring everyone home.

Thanks to everyone for a successful IOP!

SOLPEX IOP5: Lake-Effect Squalls

We just had a pretty good lake-effect squall move through the U and the eastern Salt Lake Valley.  A lake-effect squall is a burst of moderate or heavy snow accompanied by gusty winds.  It shows up in the loop below as an area of enhanced radar reflectivity with a maximum of over 40 dBZ (yellow) later the loop.  You see these all the time downstream of the Great Lakes, but less frequently in our part of the woods.  Strong flow aloft, which can be tapped into by the convective downdrafts, is perhaps a contributor in this instance.

A nice tight confluence/convergence zone persists over the southeast portion of the Great Salt Lake and northern Salt lake Valley.  This event just won't give up.  Would love to see it bubble up again.

SOLPEX IOP5: 1400 UTC View From Above

The enhanced visible and radar composite image below shows quite nicely the clear skies over and upstream of the western half of the Great Salt Lake, and the widespread cloud cover over the southeast corner at 1400 UTC (0800 AM MDT).

The north end team has been napping in their tent, but should emerge shortly to a spectacular morning.  Kudos to them for sticking it out another cold night in the middle of nowhere.

The edge of that cloud band lies between the strong WNW flow over the western portion of the lake and the NNW-NNE flow over the northern Wasatch Front.

Understanding these local low-level flows, including their underlying dynamical processes, may be a key piece of the lake-effect puzzle.  Local heating over a warm body of water can drive circulations of this type, but an added complexity here in Utah is topographic blocking and thermally driven flows produced by the terrain.  The data we collected overnight will help us guide modeling efforts to better understand these processes.

SOLPEX IOP5: Alta-Collins Lake Effect

With the transition to lake-effect occurring at around 0700 UTC (0100 AM MDT), it appears that about 4" of the snow and .46" of the snow-water equivalent that fell at Alta-Collins overnight was lake-effect related.

That's a fairly high water content snowfall (11.5%), which I suspect could be related to graupel formation from the convection.  We'll have to see if our friends at Alta can post up some snow crystal images from Tim Garrett's camera system onto their snowflake showcase.

Update 9:05 AM: Just got a note from Alta and they report a round of graupel with the front and then again from 2-4 AM with the stronger lake band, with "wind-whipped" crystals between.

SOLPEX IOP5: Nice Lake Effect Sounding

The 1200 UTC (6 AM MDT) sounding from the airport is in and is typical of a lake effect events with steep lapse rates through 500 mb and a sliver (56 J/kg) of CAPE.

This sounding shows, however, that a single profile of wind might not be representative of what is happening around the lake in the low-levels.  Perhaps the surface flow is northerly at KSLC, as evident above, but we've seen dramatic differences in wind direction and speed around the lake over night.  It will be very interesting to compare this sounding with those from our two sounding teams.  That should help to better reveal the 3D kinematic structure of these events.

SOLPEX IOP5: 1240 UTC Update

Although less widespread than earlier, lake-effect convection continues to pop up over the southeastern portion of the lake.

The area of action lies near confluence between westerly flow over the western portion of the lake and northeasterly flow along the Wasatch Front.

This flow pattern occurs frequently during these events and is not well understood.  I'm looking forward to seeing the sounding from our east side site which should provide some insight into the depth of the northwesterly flow, which may be related to flow blocking by the Wasatch Mountains.

SOLPEX IOP5 Lake Effect

Mother Nature has brought the goods, bringing us a fairly active night.  We collected upstream soundings at 12:30 am and 3 am and an east-side sounding at 3 am, and are in the process of prepping and launching the 6 am soundings.  This has the potential to be the best IOP thusfar.

The long loop below shows the development of a band of precipitation near the east shore of the north arm of the lake, which moves downstream and intensifies over Farmington Bay through about 0730 UTC.  Periods of lake-effect banding and convection continue over this area through about 1030 UTC, at which point the coverage and intensity of the lake-effect features decreases.

Just before lake-effect development, surface winds around the lake, including on Promontory Point (W@5 knots), were remarkably light.  Gunnison Island was the only site with an appreciable flow (WNW@15 knots).

Over the next hour, WSW-SW flow began to develop over the western half of the lake, as indicated by the Lakeside Mountain, Gunnison, Hat, and Badger Island Mesonet sites (Thanks DAQ for getting your data to Mesowest!).  In contrast, the flow remained light and out of the SE over much of the northern Wasatch Front from Bountiful to Ogden.  Perhaps band formation occurs on or near the boundary between these airstreams.

At the 0900 UTC sounding launch, the flow over the western half of the lake had further intensified, with a remarkable 35 knot WNW wind at Hat Island.  Promontory Point was now out of the NW@20 knots.  Winds remained light near Ogden, although the convergence line may have shifted toward the east a smidge as the flow was NW@15 at Antelope Island and WNW@10 near West Haven.  If we are lucky, the east side sounding went up in the cooler light flow near Ogden.  What a score if it did!

Over the last three hours, ending at the time of the 1200 UTC sounding launch, the surface flow over the northern Wasatch Front has gradually increased and become NW-N.  A healthy confluence line remains, however, over the Great Salt Lake.  It's unclear from this  quick analysis why the lake-effect is weakening, although it could be due to changes aloft (the soundings should help answer that question).

Mesowest sites at Hat Island and Roy (thanks to a citizen's weather observer) illustrate the differing thermodynamic and kinematic conditions on the west and east sides of the lake (technically, Hat Island is on the west shore now that the lake is so low).  At Hat Island, temperatures remained quite near ~42F following the evening frontal passage.  A remarkable veering of the winds occurred in the wake of the front, as the flow shifted gradually around the entire compass.  There was, however, a bit more abrupt transition just after 0700 UTC when the flow increased dramatically.  This may have contributed to or been a response to the development of the lake effect bands.

At Roy, which is located not far from our east side sounding site, one also sees the temperature fall accompanying the frontal passage (curiously this fall was less abrupt here than elsewhere), followed by a stabilization of temperatures at or just below 40F.  Winds here, however, are quite light during the lake-effect period, and out of the S, SE, or E until just after 1000 UTC when they become NW and increase in strength.

This could be a very interesting event to diagnose.  I did not discuss the large-scale here, but the large-scale surface pressure gradient may be an important component of driving the strong westerly flow over the western half of the lake.  

SOLPEX IOP5 Possibilities

We have teams leaving shortly to the north end and east side observing sites around the Great Salt Lake in case we get a period of lake effect tonight.  Lake temperatures are near 9C, and forecasts are for a moist airmass with a -9C 700-mb temperature to be over it by 3 AM tonight.

So, there is a brief period from about 3-6 AM where perhaps something will bubble up.  The NAM model sounding shows the moisture and northwesterly flow.

This is a fairly marginal event, as they all have been this spring.  The period of potential lake effect is brief and, instead of unidirectional northwesterly flow from the surface to the middle troposphere, there's a bit of a westerly component at low levels.  Our plan is to do a 3 am sounding launch, and then do a 6 am and possibly a 9 am launch only if something happens.  This may be our last chance of the spring, which is why we're going to be out tonight, but if nothing develops, we'll save the soundings for fall and will only lose a night of decent sleep.

SOLPEX IOP4 Concludes

The 1500 UTC soundings are in and the teams are coming home.  The broad area coverage southeast of the lake is beginning to break down and weaken and we expect this to continue with surface heating.

Although this was a weak event, we got some nice data during broad-area coverage formation that will contrast nicely with the afternoon decay observed during IOP3.

Thanks to everyone who helped out with the IOP.  I'm looking forward to a nap today.

SOLPEX IOP4: Broad Area Coverage

Over the past couple of hours, the event has transitioned into broad area coverage that is presently confined to the southeast of the Great Salt Lake.  Precip rates are weak in the valleys (virga or light snow), with mountain snowshowers.

The Mesowest surface plot shows SW flow at Hat Island, W at Gunnison Island, and WNW at Promontory Point.  At 7 AM, the flow remained westerly on Mt. Baldy.

We just can't get the flow around to NW, which is too bad as perhaps this would have been a stronger event in the valleys if we had a bit more lake modification.

SOLPEX IOP4: KSLC 1200 UTC Sounding

Steep lapse rates in the morning sounding from KSLC, but note that the winds below 700 hPa are WNW.  It would be nice if they would align more along the lake axis!  

Source: NWS/SPC

SOLPEX IOP4: 1200 UTC Update

We're in the process of launching the 1200 UTC soundings.  Over the past couple of hours, the strongest and most persistent radar echoes have become increasingly confined to the area from North Salt Lake to Ogden.

The Mesowest surface plot shows strong westerly flow over the western side of the lake, including Promontory Point.  Temperatures around the lake are now a few degrees cooler than found at Hat Island, although the lake-land temperature difference is still rather small.

It's quite remarkable that the flow refuses to go around to northwesterly at low levels.  Promontory Point remains WNW.  Even Mount Baldy never came around to NW following trough passage and remained westerly all night.

This may be due to the strong zonal pressure gradient that lies over the state (not shown).  This westerly flow was better anticipated by the GFS than the NAM, as illustrated by the soundings below.

This highlights one of the challenges of lake-effect forecasting as this contrast in wind direction makes a big difference in over-water fetch and the location of precipitation.