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Entries by Brad Rousseau (8)

A look ahead at the 2015 chase

After having a couple of tough first chase days I'll take a look at what the current situation is and a look at the next few setups in the week or so to come. Our first two chase days were tough. May 9 was our second travel day which we usually won't chase on but rather get into position for the next days chase. But given it was a moderate risk as per the SPC and was within distance for us to get to we had to give it shot. But the early start to the convection killed the chances we though there was for southeastern Kansas and we ended up chasing a messy little cell near the OKC metro. Already breaking the rules I set out for myself. 

Our first full chase day on May 10 was frustrating as well. Early morning convection had essentially ruined the environment for the risk area and split it in two. We saw the outflow boundary that surged south out of the morning convection into Texas and expected it to continue to push southward ultimately ruining the environment. An oversight that I had made was the fact that the sun was out and was able to 'cook' the boundary which allowed it to slow down and stall along the TX/OK border. This resulted in some nice supercells and a destructive tornado. We chose to head into southwestern Kansas where we thought the clearing was going to be enough for the environment to recover. But ultimately the heating was not enough. Storms did initiate but struggled to to mature. 

Today is the first of two down days. Looking at the the 500 mb and the surface chart below we can see why. An area of high pressure has setup over the central U.S. thanks to the upper (500 mb) trough pushing off to the northeast with an upper ridge centered over the western U.S. 

Thanks to this scenario we can see that the surface front has reached all the way to the Gulf shores with the warm, moist, and unstable air getting into the eastern U.S. and into southern Ontario. The second image below shows the meso analysis of the moisture and current fronts. Take note at how the surface cold front has shut down all moisture from the Gulf .


Wednesday May 13, 2015

I'm going to skip over Tuesday since high pressure will remain in place and is expected to be another down day. Wednesday looks to be our next potential chase day, although there are still many unknowns and will have to wait until tomorrow to make the final call. The setup is as follows

At 500 mb most models have a weak trough ejecting into the Texas panhandle. Note that I will use the GFS just to give visuals of the forecast just for consistency from the medium to long range as I cannot post the ECMWF forecasts and the NAM ends but Thursday evening. I will discuss the other models howsoever. For Wednesday there is good agreement with the upper pattern but there are some timing issues. The ECMWF is a little fast ejecting the trough early afternoon where the NAM is lagging a little behind the GFS. This is system is likely to be what is refered to as a "sacrificial system". This means that will will draw the front northward allowing warm moist Gulf air back into the plains. As the front is drawn northward through the day early showers and thunderstorms are possible which would ultimately kill any strong storm potential unless the precipitation can clear early enough to allow the atmosphere to destabilize.

At the surface we can see how the frontal boundary is beginning to push back north which is allowing the Gulf moisture back into Texas. In response to this trough is a weak surface low along the New Mexico / Texas borders where a weak and subtle dryline looks to mix eastward. SHOULD there be enough clearing or little morning showers than the area along and south of the warm front and along / ahead of the dryline could produce the risk of a few thunderstorms. The weaker flow aloft will not produce a tone of shear but current numbers are just on the lower threshold for weak supercells.


Thursday May 14, 2010

Starting again with 500 mb the trough for Thursday ejects into the NE / SD / IA regions with a broad upper low and speed max pushing into the Pacific coast. The ECMWF / GFS have good agreement with the setup with the NAM a little more meridional with the flow associated with the upper trough over the northern Mid-west.

At the surface the warm front will push into the north central Mid-west with a weak dryline likely to push into western Kansas and possible into western Oklahoma. The green line is where the GFS looks to have a confluent axis draped across southeastern NE, eastern KS, and into northern and western Oklahoma. Under the stronger 500 mb flow both the NAM and GFS have little CAPE. But along the southern end of the confluent axis in southern KS and into OK and along the dryline in OK the NAM and GFS are showing 2500-4000 j/kg of SBCAPE. I will note that within the moisture fields the NAM and ECMWF do have a better dryline surge into western OK. The down side here is there is weak flow aloft but wind profile do have sufficient veering winds. Its difficult to pin point where convection will be but it is possible that both the confluent axis and dryline have a low chance of producing some thunderstorms. The hope here is that the big CAPE will allow for updrafts to stretch the column so that any shear can really be enhanced. But again this is a very low risk for the time being.

 I'll leave it this for now and I will touch on the weekend potential tomorrow as a severe weather outbreak is possible for the upcoming  weekend May 15-17.

Southern Ontario Thunderstorm Potential - April 13

Looks like a real shot of mild air will push up into the region today thanks to a large amplitude upper air pattern. Below we have the Rap mesoanalysis of 500 mb geopotential heights and vorticity from 12Z (8am) this morning. The ridge that is sitting up through Québec is allowing for a good push of mild air to push up from the U.S. We can see there is a subtle trough pushing through this morning which is bringing the scattered cloud cover along with some isolated showers for the Bruce Peninsula and Manitoulin Island. There is a broad trough sitting through west central Canada down through the north central mid-west U.S. with and embedded shortwave trough sitting through Minnesota / Iowa. This shortwave will provide upper level dynamics and geopotential height falls across southern Ontario this afternoon which will aid in overall ascent across the region.

Looking up to 300 mb there is a strong 130-155 kt jet streak nosing into the Lake Superior region. This jet streak is expected to push eastward through today although expected to weaken some as it does so. This jet streak will push into southern Ontario which should help with upper level divergence as well.

At the surface from 12Z (8am) this morning, the cold front is currently sitting just along the western shores of Lake Michigan. I’ve also analyzed dew point temps in increments of 5C. We can see that the better moisture is still sitting south of the province, but a persistent 30-40 kt 850 mb southwesterly low level jet will help advect some of the better moisture into the province. Notice in western MI where the moisture is nosing in just ahead of the cold front where the LLJ is strongest. Thus we should be able to get near 10C dew points into southwestern Ontario. The Detroit 12Z sounding does show a good bit of dry air and has a convective temperature near 91F (~33C). So right away I have concerns as to how much instability we can get as moisture advection and due points being advected in will not help much. The hodograph and winds profile however is very interesting and I’ll touch more on that below. Current speed of the cold front has it pushing onto the Lake Huron Shores near 2-3pm and should begin to push into eastern Ontario this evening near 7-8pm.

The 9Z RAP forecast below shows surface T’s, winds, and pressure on the left with the 500 mb geopotential heights and winds on the right. The RAP timing of the front roughly agrees with the current speed. Thus I believe the timing for precipitation or possible thunderstorms development near 2-3pm for Windsor up the Lake Huron shore and through southcentral Ontario 4-6pm. Take note that we can see the 500 mb flow is slightly less than a 45 degree angle. So I think shear vectors could allow for some more discrete cells at first but as the 500 mb trough swings east catching up to the front the shear vectors will make for line along the front. This will be reinforced by the cold air form the precipitation itself.

Below are the NAM forecast soundings for London for 2pm and 6pm. We can see at 2pm that there is some capping in place and moisture is getting better. But still not enough to allow for any measurable surface based CAPE. But take note of the winds profile and hodograph. There is suitable veering winds with a not bad looping hodograph. As we move ahead to 6pm we can see that there is some surface base CAPE near 266 j/kg with elevated CAPE values near 370 j/kg thanks to the drastic increase in moisture with the precipitation immediately along the cold front. Thus if storms develop a little ahead of the front, we could see a few weakly rotating updrafts with some more organized storm structures initially before it lines out.  This will be farther reinforced with surface to 500 mb bulk shear values expected to be near 50-70 knots. Although some weak rotation is possible there is no substantial torando threat. Should the storms develop right along the front, than will line out almost immediately. Given the sounding along the front gusts could approach the severe limit of 90 km/h but I’m giving a low risk of that in line with the SPC’s marginal risk.

Ontario storm potential April 9 to the morning of April 10, 2015

With the SPC issuing an enhanced risk for southern Ontario lets take a step back and look at what the potential actually entails. Lets take a step back and see what the setup looks like. Below shows the 8:00 pm EDT (00Z) 500 mb geopotential heights and vorticity (or upper level spin). What we have is a broad trough sitting over the western U.S. with several embedded short waves. These short wave troughs will become key a little later on.

At the surface we have out stationary/warm front draped across much of the mid-west, just south of the Great Lakes into the central plains with a lee cyclone (low) setting up in CO. This low will push east northeast tonight through tomorrow along with the associated cold front with the warm front gradually pushing northward  through the day.

Through the morning the surface front will still sit just south of the Great Lakes with the 925mb front pushing north. As that occurs there will be 500 mb pushing into southern Ontario which is circled in red on the image below. Forecast soundings for this time do show very subtle hint of some elevated instability thus some showers through the morning are expected with a few lightning strikes but nothing much else. This first wave will also introduce a good deal of mid-level cloud across the the province which will also be reinforces by the the second upper wave (circled in black) as it approaches the province as well. Thus the potential for day time heating is quite low.

Below we see the top two images show surface and 500 mb geopotential height and the bottom is 925mb Theta-e and with surface based CAPE. We will be well into the warm sector but notice how temperatures are still quite cool. Since out large scale synoptic flow will be from the east southeast coming off of Lake Erie and Lake Ontario. This will lead to the surface air being heavily modified by the lakes and thus hinder much warming along with the cloud cover as well. This is also reflected in the bottom right image which shows surface based CAPE. No surface based instability is forecast to make into southern Ontario. But looking at the bottom left which is 925 mb Theta-e we can see some decent values pushing into the province with the leading edge of the good theta-e values along the purple line. These values indicate that there is the presence of instability aloft and near the 21Z time frame forecast soundings around SW'rn Ontario do show elevated cape values near 200-450 J/kg. As the second upper wave interacts with the leading edge of the higher 925mb theta-e gradient some scattered weak T-storms are possible. Circled in purple is the 3rd upper wave that is expected to swing through in the early evening near 00Z or 8pm EDT. The forecast soundings below are for 00Z and we can see through south central to southwestern Ontario we will have near 400-800 j/kg of elevated cape again. CAPE values are within the red squares. Notice all soundings have an inversion around the 950 mb level. Thus with the 3rd wave some more robust or organized storms are possible late day through the evening but are NOT expected to be severe by any means as no storms are likely to be rooted in the boundary layer. Thus the main risk would be some heavy downpours and intermittent lightning.




Even assessing the 4km NAM for the 03Z time (11pm) it shows some more robust convection which will be associated with the 3rd wave. But again will likely all be elevated and non-severe.

Finally we have the cold front that comes through late Thursday night into early Friday morning. The bottom left image in the 4panel below shows again 925 mb theta-e. On it we can see a very sharp gradient associated with the cold front. Ahead of it we can see modest values which are indicative of moist unstable air. The top right shows surface based CAPE with the bottom right showing MUCAPE( Most Unstable CAPE). The MUCAPE field is higher here near 100-750 j/kg because again all the instability is elevated. The sounding below is from Wiarton which is closest to the temperature peak. At 06Z or 2am the sounding here peaks in instability with 1046 j/kg but again notice we have an inversion from 950 mb up to 850mb. But with the instability in place and a sharp cold font, some of the stronger winds aloft will likely be mixed down. Thus its not until the late overnight / early Friday morning hours where we will get into the more active period. It is during this time that with the cold front passage some wind gusts could approach the severe criteria of 90 km/h. 

Even then its difficult to know whether the winds will be associated with convection or with the front itself. The 4km NAM along the the cold front does not show much precipitation except for some scattered showers. But with the strong dynamical forcing and sharp cold front I do believe we will have a frontal squall along the cold front. With this squall will be heavy rain, some intermittent lightning, and the risk of some gusts possibly getting near 90 km/h. 

So the take away here is yes there will be the risk of scattered thunderstorms through the day tomorrow. But it is quite likely that any storms will be elevated in nature and will not pose a severe threat. Any severe threat would be late Thursday night into Friday morning along the cold front itself where there will be a high wind risk.


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