ATM 211 Forecast Discussion

Case #0 – 6 March 2008

Introduction:

• Case #0 gives you the opportunity to get your feet wet with forecasting of an archived event.
• Understandably, the whole process is new to most of you, and may seem like information overload.
Fortunately, we (Mike, Joe, and Jay) take Case #0 as an opportunity to establish some ground rules, rules of thumb, guidelines etc. You’ve been given the 850-hPa method, and you’ve been analyzing maps now for over a month… now it’s time to apply everything you’ve learned in forecast mode.

Concept of these Map Discussions:

• The goal of these map discussions will be to analyze the period’s 500-hPa and MSLP analyses to understand the physical concepts behind the events that took place.
• Case #0 will be treated as an introduction to the method behind:
• Forecasting using the 500-hPa Geopotential Heights and Absolute Vorticity
• Forecasting using the 1000–500-hPa Thickness and Mean Sea Level Pressure.
• ***Always remember: If something is changed in the atmosphere, it will compensate to restore itself.***

The big deal about 500-hPa, Absolute Vorticity, 1000-500-hPa Thickness and MSLP

Why do we look at the 500-hPa?

· 500-hPa is the level where quasi-qeostrophic (QG) theory becomes applicable.

· QG simple means the winds are nearly parallel to the height contours

· QG theory makes dynamical (e.g. equations) analysis of atmospheric flow easily (arguably) comprehendible.

Why absolute vorticity?

• Absolute vorticity is the sum of the earth’s vorticity (Coriolis) and the relative vorticity at a location. The two most common types of relative vorticity are shear vorticity and curvature vorticity.
• Remember QG theory… if you change the vorticity in a location, you change the structure of the atmosphere in that location. How? à

500-hPa geopotential height AND absolute vorticity:

• ***Important content***: QG height tendency equation and absolute vorticity:

• Cyclonic absolute vorticity advection produces geopotential height FALLS.

• Indicates the forward movement of a trough of lower height/pressure.

• ***Important content***: QG omega equation and absolute vorticity:

• Differential (top > bottom) absolute vorticity advection produces rising motion!

• Rising motion assists condensation. And precipitation.

Why 1000–500-hPa Thickness?

• The 1000–500-hPa thickness is mathematically equivalent to the mean temperature in the lower troposphere.
• If the atmospheric layer is thick it implies its warm.
• If the atmospheric layer is thin it implies its cold.
• Remember QG theory… if you change the temperature/thickness of a layer at a location, you change the structure of the atmosphere in that location… How? à

Why MSLP?

• The MSLP distinguishes regions of lower pressure from higher pressure.
• Low pressure typically signifies clouds and precipitation due to low-level convergence and forced upward motion.
• High pressure typically signifies clear skies and stability due to low-level divergence and sinking motion.

1000–500-hPa Thickness and MSLP

• ***Important content***: QG height tendency equation and Thickness/MSLP:

• Warm advection produces geopotential height FALLS below and geopotential height RISES above.

• Indicates the forward movement of a trough of lower height/pressure.

• ***Important content***: QG omega equation and absolute vorticity:

• Warm advection produces rising motion!

• Rising motion assists condensation. And precipitation.

Case #0:

The general forecast issue for Case #0 is the progressive west to east movement of a cold front across the US that triggered moderate precipitation (Category 4) totals along the way.

Cities:

ATL, BNA, MEI, MSP, ORD, STL

Monday

• On the MSLP map the important feature is a complex region of low pressure along the lee of the Rockies with primary centers in the south central plains of the US and southern Alberta on Monday.

• These regions of lower pressure were located downstream of 500-hPa trough and absolute vorticity maxima.

****Manifests the idea that systems in the atmosphere tilt upstream during their cyclogenesis phase.*****

• The upstream tilt ensures differential cyclonic absolute vorticity advection over the low pressure center. RISING MOTION. Compare these regions to the observed regions of precipitation on your analyzed maps.

• The 500-hPa trough digs throughout Monday and into Tuesday. The answer why is found upstream. A cyclone (see MSLP) and large region of cyclonic vorticity advection (and warm air advection) is present over the Gulf of Alaska and into British Columbia.

****This amplifies the downstream ridge that, in turn, digs the downstream trough. Termed downstream development****

Tuesday

• Cold air pushes southeastward (1000–500-hPa thickness minimum) and marks the boundary of the cold front.

• 500-hPa vorticity maximum and MSLP low pressure center become collocated. System is vertically stacked. Compare to 12Z and 18Z surface map.

****As a system becomes vertically stacked it begins to occlude. Justification for occluded front 18Z Tuesday.****

Wednesday–Friday

• 500-hPa trough and MSLP minimum move toward the northeast over the Canadian Maritimes.
• Region of cyclonic vorticity advection aligns along Appalachian Mountains and lifts northward.

• MSLP trough denoting location of the cold front reaches the coast by 12Z Wednesday and offshore by 00Z Thursday.

• Upstream region of absolute vorticity “hangs back” and is picked up by the next digging trough over the Rocky Mountains by 00Z Friday.