Differential Cold Air Advection

Hey everyone... Almost 2 years ago, it was my 3rd day working at the NWS Des Moines office. We had a blizzard come up without any lead time, and it closed major roads and killed 7 people in Iowa. However, it certainly didn't come "out of nowhere." This spawned a research study to better understand and forecast these event. As luck would have it, we had a similar event this past week, so I thoughts I'd compare/contrast. First of all, to crudely quantify the impact, I'll show a map of the road conditions. This first one is from Jan 26 2010.The next one is from Jan 11 2012.
the Jan 26th one must have been worse right..? Well, kinda, but in my opinion that is due to one reason. SNOWPACK. Jan 2010 had nearly record snow pack (1-2ft) across Iowa, whereas this year we had a whopping zero. No snow pack means no snow to blow around, and any falling snow will gather in the grass, fields, and ditches. To prove my point, here's the metars from Mason City Iowa (think north central Iowa).

Jan 11 2012
time temp press VISBY Wind Gust Weather

3:17 PM	26.6 °F		29.75 in	1.8 mi	NNW	27.6 mph	38.0 mph	Light Snow
3:53 PM	25.0 °F		29.80 in	1.8 mi	NW	25.3 mph	38.0 mph	Light Snow
4:53 PM	21.9 °F		29.82 in	1.8 mi	NW	26.5 mph	36.8 mph	Light Snow
5:53 PM	19.0 °F		29.83 in	2.5 mi	NW	32.2 mph	46.0 mph	Light Snow
6:53 PM	18.0 °F		29.85 in	3.0 mi	NW	32.2 mph	44.9 mph	Light Snow

Jan 25 2010
time temp press VISBY Wind Gust Weather

10:53 AM	21.9 °F	29.38 in	0.8 mi	NW	33.4 mph	41.4 mph	Light Snow
11:53 AM	19.9 °F	29.39 in	0.5 mi	WNW	29.9 mph	40.3 mph	Light Snow
12:53 PM	19.0 °F	29.39 in	0.5 mi	WNW	32.2 mph	40.3 mph	Light Snow
1:53 PM	18.0 °F	29.41 in	0.2 mi	WNW	32.2 mph	48.3 mph	Light Snow
2:53 PM	16.0 °F	29.45 in	0.2 mi	WNW	36.8 mph	44.9 mph	Light Snow
3:53 PM	14.0 °F	29.49 in	0.5 mi	WNW	26.5 mph	38.0 mph	Light Snow

So whats going on...? First of all, the pressure is rising, which doesn't fit the classic blizzard conceptual model. Second of all, there's only light snow being reported. Thirdly, the northwest winds are impressive. These three things are all common with strong cold fronts. However, the question I had is "how does strong wind mix down to the surface when surface Cold Air Advection is only going to act to stabilize the boundary layer..? The answer my friends is to replace "Cold" with "Differential". Differential Cold Air Advection (usually between 925mb and the Surface) will steepen the lapse rates in the boundary layer and promote momentum transfer of strong winds down to the surface. Here's the 12Z (pre-frontal) and 24Z (post frontal) soundings from Omaha during this last event on Jan 11 2012. The black Xs on the 12Z sounding show the 850 and Sfc temp from the 24Hr sounding (dtemp/dt). This was unusually deep mixing in the boundary layer. I also circled the 0-1km wind shear. (its blurry, so click it)

What's going on with the weather over the past few months?

Hey fellow chasers and weather enthusiasts,

It's been a while since any of us has posted to the blog and wanted to add some recent discussion brought up by Jake concerning the unique weather pattern most of us have experienced over the past few months. Jake's opened ended question has opened up some great discussion between himself, Dima, and I along with many forecasters around the country. Here are a few of the emails that have been sent thus far and I will add Dimas response after he has time to send out his response:

From Jake Beitlich:
In your own words...what is going on this year...? It seems like this entire winter has been "northern jet stream across southern Canada, with a cutoff low over the desert southwest." If you get a chance, could you please explain this in 101 terms concerning the NAO, PNA, and La Nina?

Here's my take.....
- La Nina means weaker than normal subtropical jet...therefore the southern stream has a harder time phasing with the northern stream (a.k.a cutoffs over 4corners that slowly meander eastward toward Louisiana and don't emerge as the panhandle hooker as major cyclones.)
- Meanwhile strong westerly flow over Alberta...aka lee side warming, keeps the cold arctic air to the north (not sure how this is anomalous from previous winters).
- I don't exactly know how the NAO and PNA are related.....
- I also don't know how to explain why this 500mb image I sent seems to be the most common weather map of the season.
Looking forward to your thoughts,

My response:
I love this question Jake. This exact question has been the talk of the weather community here in the Desert Southwest for the last 3 months. I’ll try and elaborate on this topic as best I can (a limited understanding of the underpinnings of the large scale teleconnections at work, ENSO, MJO, NAO, PNA, and the AO).

Building a little on Jake’s take with the help of some colleagues:

1. Weak La Nina phase of ENSO: It’s well documented that this past summer/fall into the start of the winter season has been dominated by La Nina conditions especially noted over the Nino 3.4 region (central equatorial pacific). Typically moderate to strong La Ninas favor upper level ridging over the western half of US (drier and warmer conditions) with large scale troughing over the central/eastern US (cool and wetter conditions). However, weak La Nina to ENSO neutral conditions (like what we are currently seeing) complicate the large scale pattern and can allow for periods where the western ridge breaks down for periods of time during the winter months. The climatological storm track across the Western US during the phase mentioned above is more conducive to a prolonged cut-off low season across the southwest.

2. Madden Julian Oscillation (MJO): 30-60 day Tropical Kelvin Wave that traverses the equator with strongest signal in tropical Pacific during weak La Nina/ENSO neutral conditions. Can have significant impact on the upper level storm track across the Western US. Impacts include large scale variation in temperature, winds, and precipitation patterns which are similar to moderate El Nino but with a shorter duration. I’ve included a figure from a presentation that Mike Baker (NWS Denver/Boulder) gave back in December showing the November-December storm tracks which were possibly aided by a positive phase in the MJO.

3. NAO and PNA: Going to defer to Dima on this one as I don’t really know enough to make any strong points. About all I can say especially towards the NAO is that it blends with the Arctic Oscillation (what I discuss in limited detail below). Dima I know there is large disagreement within the climatological community as to which oscillation plays a more dominant role and thus gives a more representative depiction of the large scale/dynamical pattern.

4. Arctic Oscillation (AO): Arctic climate index with positive and negative phases that represent the mean upper level pattern over the Arctic Latitudes. In the positive phase, generally lower than normal pressure over the polar region and higher pressure over mid-latitudes steering the majority of Pacific storms northward. Frigid winter time temperatures don’t extend as far south into central US. The negative phase is a general reversal of the pattern just described. Our buddies over at the NWS PHX office had a nice little presentation covering this a few weeks back. I’ve attached a slide from their presentation that gets more into what a Positive AO means for the Southwest.

My General “101 style” take away thoughts (please feel free to correct me if I’m making any misguided statements):
- Weak La Nina to ENSO neutral conditions favor sporadic but prolonged breakdowns of the western ridge normally seen during more moderate La Nina type winters.

- A Positive MJO index (November-December, now slightly negative) aided in transport of moisture rich tropical air across southwestern US by way of subtropical jet and enhancement of Western Pacific troughing pattern during that time.

- A positive AO which favors low pressure over Southern California in November and December despite the bulk of Polar Jet energy being steered into Central and northern Canada i.e. all of the cut off low scenarios we’ve seen the last 60 days. A positive AO also typically keeps temperatures much warmer and conditions drier to the east of the Colorado Rockies which we’ve also seen across the Plains and Midwest (it’s unbelievable how warm it has been thus far).

As best as I can understand it in terms of the large scale pattern, it seems like a pretty unique coupling of several dominant teleconnections has allowed for our interesting start to the winter. I honestly can’t remember seeing anything quite like this in the 10 or so years of weather forecasting that I’ve been a part of!!!

I’m really looking forward to hearing our expert Climatologist’s thoughts. It’s such an interesting topic and directly related to forecasting challenges Jake and I have seen over the last two months.

This post will be updated in the coming days as further responses become available:

Dan aka Squall Line

Can nasty dewpoints be useful?

Have you ever sat around when its 95 over 82, with 6000 J/kg of CAPE laughing at you, frustrated why you can't buy a cumulus cloud, much less a thunderstorm? Well, I have. And since we had quite a few of those days this summer, I did a bit of thinking. Two questions:

1. Why do you never see temperatures well above 100 with dewpoints of ~78F+? Note that all the extremely hot days in the southern plains usual see 100F+ temperatures with dewpoints 70F or lower.
2. In the summer, why is it that you can get very high dewpoints without even a cloud, while other areas with lower dewpoints see storms?

To answer both of these, I'll use a simple equation that explains dewpoint tendency within the boundary layer:

Now let's butcher this equation...

The first term is the dewpoint tendency, because we're talking about those nasty days where the dewpoint stayed at 81 or whatever all day, term (1) is 0. Also, many of those days were fairly calm, so the horizontal advection of dewpoint, term 2, is negligible. In fact, if we're trying to explain 80+ dewpoints, then this term can only be detrimental to us, because dewpoints all around are likely equal or lower than ours, so this can only be a sink term. Thus, all this simplifies down to 2 things, upward motion and plant breathing. Plants consistently add moisture to the air, so term (3) turns out to be the most efficient way to lower the dewpoint on a calm summer day...

Finally, this brings me to the conclusion: the reason why we don't see thunderstorms with 80+ dewpoints is because of the dewpoints themselves! They are telling us that the cap is incredibly strong so there's hardly any mixing going on with the free atmosphere (even though the boundary layer itself is growing in height). This may also explain why it's hard to get 100+ temperatures with 80+ dewpoints. In order to get 100+, there almost has to be mixing down of the high-theta air that resides within the cap...

Anyways, that's two cents for the day.

Dima

Heat Wave - Martha and the Vandellas Style

Well hey there!

Since no one is posting or chasing because the dog days of summer are about to set in, I decided to post an 850 hPa map from GFS 1200 UTC 14 July 2011 to show the world how much fun everyone is going to have during this heat wave! This is a plot for 120 hr ahead (0000 UTC Fri):



Thank you eastern US ridge!!! Time to drive up the a/c utility costs!

Helicity

UW Storm Chasing - Alumni Edition

What up team UW-Madison,

While chasing today in the US is most likely going to involve chasing some sort of QLCS or MCS or something, we will be sending out a chase car to see if we can bag a tornado during the middle afternoon hours before everything goes to H-E-double hockey sticks. I describe some advantages and disadvantages below, and note that this discussion is brief and very casual, so if you want to comment more on it go for it, that's what this blog is all about baby:

Chasing Advantages:

Wisconsin located in the warm sector, near the triple point of the deep low pressure system. An occlusion will develop, with the warm sector branching from the twin cities clockwise to Madison and into Illinois. Warm, moist air will be advected from the Gulf of Mexico region into the upper Midwest, and this along with daytime heating will allow for the breaking up of a very weak (if at all even one) EML and morning cap over eastern Iowa and into south-central Wisconsin and destabilize the atmosphere this afternoon. SBCAPE values will reach 1500-2000 J/kg this afternoon. In terms of shear, the region exhibiting maximum helicity (both 0-1 km and 0-3 km) occurs near the warm front, which will push northward into eastern MN and central WI by 1800-1900 UTC. This, along with strong upper level forcing associated with the approaching upper level trough will allow for development of some respectable convection, and hopefully some discrete cells will fire between the triple point and near the warm front starting ~4 pm local time (2100 UTC).

Also, we'll be in Wisconsin, so the home crowd is on our side (that means wear the red AOS "home" jerseys and khaki shorts or else...)

Chasing Disadvantages:

Well, the shear profile is not the best in terms of chasing tornadoes today. Wind speeds remain southerly throughout the lower and middle troposphere east of the cold front (Illinois, southern Wisconsin, southeastern Iowa, south from Illinois towards the Gulf of Mexico), and bulk shear and helicity values (according to the 1200 UTC NAM and HRRR) are not favorable except near the warm front by I90/I94 during the afternoon hours. Since CAPE values are favorable east of the cold front and within the warm sector near the triple point, my guess is that any storms that fire will become linear and form some sort of squall line (with respect to the upper Midwest). Hence, chances of chasing tornadoes safely and accurately are not too high, except for near the warm front and triple point.

OUR GO ZONE: Tomah, WI. It will be just south of the warm front and south-east of the triple point, and is also near the I90/I94 interchange. If we get there by 1830 UTC (1:30 pm CDT), this will give us plenty of time to check out cumulus fields via satellite data (and our eyeballs) and see where to go from there. HRRR is showing lots of scattered convection and cells starting around 2100 UTC. I'm not trusting the HRRR completely, since it overestimated the strength of this morning's MCS and made the one in central Illinois look more epic than it really was. While are chances at seeing a tornado are very small, this will give us a chance to catch any supercells early on before they either die off or possibly congeal and kill our chasing chances for the evening.

Peace out,

Helicity and friends

P.S. Don't hate, use KolourPaint:

Area Forecast discussion...for Monday June 20

I don't post much...but I thought since I took the time to do this
that it should go on the Tommy HRRRRR page...I really wanted to drop
the phrase "go zone"...but didn't want to sound like I was favoring
severe weather...death...and destruction...

A WEAKENING MCS IS EXPECTED TO BE ONGOING BY 12Z TOMORROW MORNING.
THE EXACT PLACEMENT OF THIS MCS...AND THE OUTFLOW BOUNDARIES IT LEAVES
BEHIND WILL HAVE A LARGE IMPACT THE TIMING AND PLACEMENT OF CONVECTIVE
INITIATION MONDAY AFTERNOON. GUIDANCE SHOWS A SHORTWAVE MOVING ACROSS
THE IA/SD/NE BORDER EARLY MONDAY MORNING...WITH THE BEST H850 THETA_E
ADVECTION ACROSS THE IA/MN BORDER. THESE LINGERING THUNDERSTORMS SHOULD
BE ELEVATED...WITH MINIMAL THREAT FOR SEVERE WEATHER...ALTHOUGH A STRAY
WIND OR HAIL REPORT IS POSSIBLE. HAVE HIGHEST POPS ACROSS THE NORTH
TO REFLECT THE NORTHERN TRACK SUPPORTED IN BY THE QPF PLACEMENT IN
THE 19.12Z GFS..ECMWF..AND NAM. HOWEVER...EXPANDED POPS ACROSS THE
ENTIRE CWA TO ACCOUNT FOR THE CONVECTION THAT IS EXPECTED TO BREAK
OUT ACROSS NEBRASKA SUNDAY EVENING...AND MAY STILL BE AROUND BY THE
START OF THE PERIOD. 
THROUGHOUT THE REST OF THE MORNING HOURS AND INTO EARLY MONDAY
AFTERNOON...THE UPPER LEVEL PV ANOMALY ACROSS THE TX/OK PANHANDLES
SHOULD CONGEAL AN TAKE ON A NEGATIVE TILT AS THE JET STREAK ROUNDS
THE BASE OF THE TROUGH. ANY RESIDUAL CLOUD COVER FROM THE OVERNIGHT
CONVECTION SHOULD QUICKLY DISSIPATE...AND WARM AIR ADVECTION WITH
H700 TEMPS OF 14-16C WILL ESTABLISH A CAP ACROSS CWA. AN ELEVATED
MIXED LAYER FROM 700-500MB...COMBINED WITH AFTERNOON CLEARING SHOULD
ALLOW THE BOUNDARY LAYER TO RAPIDLY DESTABILIZE WITH SBCAPES OF
3000-4000J/KG IN PLACE BY LATE AFTERNOON. THIS CONDITIONAL INSTABILITY
WILL LIKELY BE REALIZED BY LATE AFTERNOON/EARLY EVENING AS THE STRONG
SYNOPTIC FORCING WITH THE HEIGHT FALLS AHEAD OF THE PV ANOMALY WILL
DYNAMICALLY DESTABILIZE THE MID LEVELS AND ERODE THE CAP. GIVEN THE
FAVORABLE THERMODYNAMIC ENVIRONMENT IN PLACE...ATTENTION THEN TURNS
TO THE WIND PROFILE TO DIAGNOSE THE CONVECTIVE MODE. BY MONDAY AFTERNOON
THE AFOREMENTIONED JET STREAK WILL BE SURGING NORTHWARD ACROSS CENTRAL
KS...EASTERN NE. DEEP LAYER SHEAR VALUES OF 50-60KTS WILL SUPPORT
SUPERCELLS. MEANWHILE 6HR PRESSURE FALLS OVER EASTERN NE/SD WILL
RESULT IN SOUTH SOUTHEASTERLY SURFACE WINDS. SOUNDINGS SHOWS VEERING
WIND PROFILE IN THE LOWER LEVELS FAVORABLE FOR TORNADOES GIVEN THAT
THE STORMS WILL BE SURFACE BASED. THE HIGHEST SHR VALUES WILL BE
ALONG THE WARM FRONT...WHICH SHOULD BE LOCATED ACROSS THE NORTHERN
HALF OF IOWA BY LATE AFTERNOON. 
CURRENTLY ENVISION THE SCENARIO UNFOLDING WITH THE WARM FRONT LIFTING
ACROSS THE I80 CORRIDOR BY 18Z...HWY20 BY 21Z...AND TOWARD HWY18 BY 24Z.
GIVEN THE MODEL TRENDS WHICH HAVE INTENSIFIED THE SYSTEM SLIGHTLY AND
SLOWED IT DOWN...DONT SEE THE WARM FRONT SURGING NORTHWARD INTO
MINNESOTA. ALONG THAT SAME LINE OF THOUGHT...FEEL THAT THE CAP WILL
MOST LIKELY HOLD UNTIL AT LEAST 21Z...WITH CONVECTION BREAKING OUT IN
THE AREA OF ENHANCED CONVERGENCE ALONG THE WARM FRONT. THESE STORMS
ARE EXPECTED TO BE SUPERCELLULAR AND COULD QUICKLY TURN SEVERE. MEANWHILE
ANOTHER ROUND OF STORMS SHOULD DEVELOP CLOSER TO THE UPPER LEVEL DYNAMICS
ALONG THE DRY LINE IN EASTERN NE/KS...AND MOVE THROUGH CENTRAL IOWA
OVERNIGHT. THESE STORMS WILL INITIATE AS DISCRETE SUPERCELLS...AND
LIKELY GROW UPSCALE INTO A LINEAR COMPLEX WITH EMBEDDED HP SUPERCELLS.
THE SEVERE THREAT MAY LAST THROUGH THE EARLY MORNING HOURS OF TUESDAY.

- Hailstone...

Few Clips of Chase video from 455

Here is a sneak peak of footage from our chase class, dates and locations are included in the videos...a more comprehensive webpage will be completed in the coming days. 

Watch in 720p!

http://www.youtube.com/watch?v=S3JZmTaUbS8

http://www.youtube.com/watch?v=xjGLOF1lSeQ&feature=related

http://www.youtube.com/watch?v=Jfc47lLzNUc

Convergence