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Ice Potential? Thunderstorms?

6 Mar 2009, 4:23 pm
Forecast Precipitation Type (3.5 Day)
Forecast Precipitation Type (3.5 Day)
Forecast Precipitation Type (7.5 Day)
Forecast Precipitation Type (7.5 Day)

Our mixed bag of tricks discussion from Wednesday certainly seems to be coming together quite nicely. For today we will be discussing the double barrel lows, one of which will be more impressive than the other, the slight push of colder temperatures in the north and warm highs for the next week in the south, then some SPC convective outlooks.

To begin with lets examine our Medium Range Forecast Precipitation Type Animation. Recall that this product is a forecast out 180 hours or 7.5 days, and each frame in the animation represents a 6 hour window beginning 6 hours prior to and ending at the valid time (represented by the timestamp on the bottom). The purpose of this animation is to generalize pressure centers and distributions (highs / lows / isobars), and infer that if precipitation were to fall within the 6 hour window, what form might that precipitation take (snow / rain / mixed).

For our 12z run, the first forecast hour (F006) is valid at 1pm this afternoon. It shows the remnants of the low we previously discussed on the border of the Canadian provinces of Ontario and Quebec. We also see the low pressure center over Kansas that we previously discussed would form. What happens when we advance a bit?

As we advance through 1am Saturday morning (F018), the baby clipper in the north is sitting at the northern tip of Maine, our low in Kansas is deepening a bit, while we also see the pressure distributions in Montana, North Dakota, and South Dakota plunging a bit, and indication of the colder air we said on Wednesday would start to slide down. We also still have some snows in the Rockies at this point, to be expected.

Now lets advance through 1pm on Saturday (F030), and we see a nice band of snow extending from the mountainous areas of Utah and Colorado, with a band extending through Wyoming, Nebraska, South Dakota, up into Minnesota. The low has dug a little deeper at this point, while our little clipper in the north is now heading to Newfoundland. Rains will have a good chance across the Midwest at this point, some in the afternoon on Saturday could be quite stormy.

As we advance through 1pm on Sunday (F042) snows are still lingering along the stretch of northeastern Colorado through central and western Nebraska, southern South Dakota, and into southern Minnesota.

By the time 1pm Sunday rolls around, the low finally starts to move to the northeast, becoming centered on northern Illinois. At this point the dividing line between snow and rain becomes a little iffier, with our medium range product showing snows still in northeastern Nebraska, northern Iowa, and southern Minnesota through Wisconsin, and into northern Michigan, including the U.P. While rains will linger to the south and west, with thunderstorms a possibility on this day as well.

Not to be left out is the Pacific Northwest. As we scroll back to 1pm on Saturday (F018), notice the baroclinicity north of Washington State along British Columbia. As we advance through our days again we see those tightly packed isobars begin to descend, with snows funneling into Washington, and upper elevations in Oregon. So that by the time Sunday rolls around snows will have invaded south into California into the northern Rockies. Now since this is not a high resolution product, it is difficult to discern from this general forecast the exact nature and distribution of snows, we will get to that a bit later.

Notice what happens on Monday at 7pm (F084). We have the second big low that we discussed on Wednesday still showing itself! Just for grins and giggles, advance through next Tuesday at 7pm! My that’s a pretty sight isn’t it! Looks like a hurricane sitting above Lambeau field doesn’t it! As we advance through 7am on Wednesday she heads off into Canada, but it will be interesting to see if she digs as deep as our medium range forecast projects. Look at those tightly packed isobars! Can anyone say winds? =)

Well now that I’m all hot and bothered by the medium range forecast, let’s compare it to our shorter range product, our 84 Hour Forecast Precipitation Animation to see if there are any differences. Recall that this product is of course an 84 hour forecast product, but at 3 hour windows of time instead of 6, also representing that if precipitation were to occur up to and ending at the valid time, what might form might it take? It is also a considerably higher resolution product. So, lets compare.

Since i just ran this again in the middle of writing, it is our 18z run, and we see the expected snows in the Rockies at F003 valid 4pm EST. Let’s advance our frames out to F015, valid at 4am EST on Saturday. Do we see a difference here to our medium range product? Of course, notice the big red blotch in Nebraska? Icing potential. Our medium range product is fantastic at gauging pressure centers and distributions, and general patterns of precipitation, but is not really designed to gauge at the level of our shorter range product. Icing is a definite potential with this system.

Now advance just one single frame to F018 7am Saturday morning. What to we see? The colder air is beginning to descend that we discussed on Wednesday, at the same time changing the temperature profile of the atmosphere (column of air) and our mixed potential becomes more refined running from southwestern Nebraska to northwestern Nebraska while snows are beginning to take shape.

Slowly advance two more frames and glance quickly at each. See how the colder air is descending and a wider band of snow is forming along the “backside” of the low? Notice that from the 1st to the 2nd frame (F024) valid 1pm on Saturday the icing becomes “pushed out” (in reality the colder air has continued its advection so that the column of air is now below freezing at higher altitudes forming snow instead of mixed) and we now have a nice band of snow extending from northeastern Colorado through Nebraska? Notice also that rains are still a possibility here as previously discussed some may be stormy. Also note the similarity between our medium range product above and this shorter range product. Our medium range product did OK in depicting the evolution of snows, but not well in grasping the mixed potential, this is due mainly to its lower resolution.

Advance now through 4pm on Saturday, the snows are still in place, and icing is now a potential again due to the warmer air advecting from the south creating a pocket of above freezing air aloft.

Now that we have the general idea we can continue to advance through 1am Sunday (F036) and see the snows we expect at this point, the potntial for icing, and rains across the same general area, but from here what changes?

Advance two more frames from 4am to 7am Saturday, and notice that our low has now “taken off” and begins moring quickly, so that by 7am it is centered basically over Kansas City with snows still in Nebraska, some in South Dakota, the northern tip of Iowa, with mixed potential still in the running.

By 1pm Sunday we have snows across most of Iowa now, still in southern Minnesota, with mixed from northeastern Iowa through Wisconsin, and rains over much of northern Illinois with a band stretching northeasterly along southeastern Texas.

Chicago seems to be the bullseye for our shorter range product for the position of the low on Sunday evening at 7pm, with snows concentrated in Wisconsin and northern Michigan with a line of mixed potential and rains in the southern extents. By Monday morning, a larger area of mixed potential is forecast with snows still lingering and rains running along the southern flank, of course, don’t forget to compare the forecast snows in the PNW!

These products are not perfect at depicting exact areas of precipitation, but are wonderful tools in understanding the evolving nature of the atmosphere in the future. I can say however, that the potential for ice is definitely in the mix on a line running from Nebraska through Iowa, southern Minneota, Wisconsin, and in Michigan. Snow totals will vary from light to 4″ along the lines described above, with the potential for more in northeastern Nebraska, and Wisconsin to Michigan will see up to around 4″ with the potential for a tad more in isolation locations. Keep an eye on things to come and see how well the animations hold up!

Forecast Maximum Temperatures (7.5 Day)
Forecast Maximum Temperatures (7.5 Day)
Convective Outlook
Convective Outlook

For the next few days the SPC has issued Convective Outlooks for the central Great Plains and heading into the midwest. View the link where you may find each outlook by day, as well as probabilistic products.

I’m not overly impressed by what i see for today, so I have not taken the time to examine the dynamics in any detail. What i see at a quick glance for today is the possibility of storms that may support hail.

As we advance through the weekend I do see a nice little jet streak moving through along with the developing low level trough that could certainly setup a nice shear profile supportive of supercells altough it does seem that hail is once again the biggest threat, though a landspout or two with the possibility of a tornado is not out of the question. If I were out on Saturday i would sit on the I35 corridor in Kansas, but I’m getting much to old for that. Hail also looks to be more in the cards for Sunday with steep mid level lapse rates, and enough shear to not rule out a tornado.

For our last topic let’s examine the Forecast Maximum Temperature Animation. This product represents a 180hr forecast (7.5 Days), at six hour intervals with each window of time representing the six hours prior to and ending at the valid time (timestamp at bottom), and shows what the maximum temperature for the entire six hour window is forecast to be.

For our first image, valid 1pm this afternoon, note the wonderfully warm air centered over the southern and central great plains. Recall our thin blue line represents the TOI (Temperature of Interest) of 72ºF. Notice the entirety of the southern plains, central plains, midwest, and almost all of the northeast is below our TOI of 32ºF, the thin red line. Only the tip of Maine is forecast to be below freezing in that area, with the usual Rockies and northern plains below freezing. As we advance to 7pm all of conus including Maine is above freezing except the Rockies (recall this represents the previous 6 hours as well).

As we advance through the weekend we see the cold air advection from the north bring the freezing line down, since this is not a high resolution product, interpret the differences of the freezing line along Nebraska for the snows that we discussed above.

The good news is, let the animation fly and what do you see? As we go through next week is the south in for trouble? Or do we see generally warm temperatures? I admit I get a little nervous at the F144hr forecast valid 7am Thursday. How far south will that cold go? We’ll see next week!

Please be sure to view our Advisory and Radar Centers for the latest updates and developments.

As always stay tuned to your favorite weather outlet, stay informed, and stay safe!



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