Forecast Bust: Synoptic Analysis
Forecasts will bust. I know this is shocking to hear, especially to my boss who announces to anyone that will listen that, "the forecasts are always wrong!" However, despite the relatively accurate nature of today's forecasts, they are certainly not immune to errors. Often these forecast busts are the product of a forecaster failing to connect the big picture to the model output. For instance, you can bury yourself in numbers from the Model Output Statistics (MOS) forecasts from the GFS and NAM models, the GFS and NAM dynamic models, and the Short Range Ensemble Forecasts (SREF), without even reviewing a upper air analysis and a surface plot analysis. Such an approach is not sound, resulting in routine forecast busts.
In this particular forecast, I predicted the low temperature for the period to be 29F, which busted as the actual low was four degrees lower at 25F.
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| Screenshot of the WxChallenge results for March 2nd, 2012 at KABQ. Note that I am udrCL9. |
So where did I go wrong? Let's backup and take a look at the big picture as it was forecasted on March 1st for March 2nd.
Big Picture
On the morning of March 1st, the surface analysis showed a stationary front northwest of the Four Corners area with several surface lows along the boundary.
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| 12Z Surface Analysis for Thursday, March 1st, 2012. Courtesy: HPC |
The HPC 30hr Surface Forecast, valid at 06Z that evening shows the once stationary front now clearing KABQ.
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| 08Z Surface Forecast for 06Z Friday, March 2. Courtesy: HPC |
HPC Surface Forecasts at 18Z Friday and 00Z show continued progress of the cold front by the end of the period, with high pressure building in behind the front.
If we look at the 12Z NAM 18hr forecast for MSLP, 850mb temperatures, and 6-hr total precipitation, we see a pattern developing that is prime for low overnight temperatures.
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| 12Z NAM 18hr forecast for MSLP, 850mb temperatures, 6 hr total precipitation valid at 06Z, March 2nd. Courtesy of NCEP |
The red and blue lines on the forecast map are 850 mb isotherms in 10C intervals. If you look at the Colorado/New Mexico border, you see a tongue of colder air at 850 mb dipping into NM. This is a product of a small area of high pressure over Colorado, but more importantly the large surface low over Oklahoma. With a northeasterly flow around the backside of the low, enhanced by the weak high over Colorado, some cold air advection (CAA) is occuring north of KABQ by 06Z. Further, the diverging surface winds over NM and weak MSLP gradient sets up the overnight period for weak surface winds.
Cloud cover is another large factor in the overnight low temperature, and for that we'll look at the forecasts at 500 mb and 700 mb.
At 500mb, with stronger values of absolute vorticity shown in shades of orange, you can see an interesting feature at the bottom of the large upper air trough that dominates 2/3 of the US. The large areas of absolute vorticity are separated at the base of the trough due to a small shortwave ridge. Below are the forecasts for 06Z, 09Z and 12Z, which were the critical period for setting up the low temperature.
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| 12Z GFS 18hr 500mb forecast, valid at 06Z. Courtesy of NCEP. |
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| 12Z GFS 21hr 500mb forecast, valid at 09Z. Courtesy of NCEP. |
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| 12Z GFS 24hr 500mb forecast, valid at 12Z. Courtesy of NCEP. |
Normally, with the proximity and placement of the vorticity maximum over southern Utah, we'd expect clouds developing over New Mexico. Downstream of a vort-max is marked by an area of upper divergence, which encourages upward motion. However, this subtle S/W ridge has the effect of upper air convergence and downward motion. With a short period of stable, sinking air, clouds would have a tough time forming. The 700 mb forecast shows the feature even more distinctly:
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| 12Z NAM 18hr 700mb forecast, valid at 06Z. Courtesy of NCEP. |
The tongue of higher RH values at 700mb pressing through the Four Corners into NM concerned me that it would introduce some clouds into the region. The 24hr forecast prog shows the higher RH values downstream of the vort max over southern Utah, but they have remained mainly in NW New Mexico.
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| 12Z NAM 24hr 700mb forecast, valid at 06Z. Courtesy of NCEP. |
That small ridge in the larger trough impacted the forecast region between 06Z and 12Z at the beginning of the forecast period, limiting clouds, which further encouraged a lower overnight temperature.
I was actually torn between when the low temperature would actually occur. For most locations, the low usually occurs in the hours just before dawn, which at KABQ would be near 12Z. But under certain circumstances, the low temperature can occur at the end of the period. With falling heights during the day, I suspected that the daytime high could fall short of the MOS forecasts and lead to lower temperatures at the end of the period.
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| 1000-500 mb thickness from 12Z March 2 - 06Z March 3. Courtesy of University of Wyoming. |
With the big picture in mind, we'll now review the actual MOS forecasts for the period, review my methodology and analyze where I went wrong.
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