I. Biases of the Short-Range
Numerical Models (Days 0-3).
- A. The Aviation Model
- Since this model is generated
from the NCEP Global Spectral Model (GSM), as true for the MRF,
the AVN tends to have a "cold" bias, mainly across
the eastern US. Remember that the AVN and MRF are the same numerical
model; the difference is in the data cut off times. The AVN will
generally display the same biases as the MRF.
- Overall, for situations when
cyclogenesis is possible, the AVN may be the first short-range
operational NWP model to capture the event. However, this model
can be too slow with the deepening rates of sea-level cyclones.
The AVN model is the best at simulating filling cyclones.
- The 1200 UTC AVN model can
be quite useful for detecting important forecast "trends"
for all the operational models. A good example would be comparing
the "latest 72-hr AVN" with "last nights 84-hr
MRF" model solutions.
- This model tends to have
the smallest position errors for cyclones and anticyclones at
days 2 and 3. In general, for sea-level lows, the AVN does have
a tendency to "track them" too far northwest for cases
of cyclogenesis to the lee of the Rocky Mountains. The latter
is especially the case when the mid/upper tropospheric trough
(helping to initiate the sea-level response) is still undergoing
amplification over the Great Basin. Often the AVN (and NGM) will
over develop a lead short-wave trough and allow its unrealistically
deep sea-level response to advance too far north and west. Once
the mid/upper tropospheric geopotential trough and/or closed
cyclone center has advanced to the lee of the Rockies, the sea-level
cyclone placement is fairly reasonable (generally a baroclinic
mode of development).
- The AVN often underdevelops
surface lows, especially over the oceans.
- This model tends to underpredict
precipitation amounts during the warm season for greater than
- There is a tendency to predict
1000-500 mb thicknesses too low over sea-level cyclones.
- This model can be too slow
with deepening rates of baroclinic cyclones, especially during
the first 12 hours.
- B. The Nested Grid
- The NGM tends to overdeepen
sea-level cyclones over land, and underdevelop them across the
oceans. This model can be too slow to fill weakening cyclones.
- The NGM has a "northern
bias" with respect to storm track prediction, similar to
the AVN model. In general, the NGM will be farthest north of
the 3 short-range operational models discussed here. In the case
of the NGM, this northern bias is also true for the eastern US.
- The NGM tends to predict
sea-level pressures to be too high for anticyclones across the
- The NGM tends to overpredict
the magnitudes of sea-breeze convergence zones, and the precipitation
- During the warm season, the
NGM has a bias to overpredict light precipitation (amounts less
than 0.50"), especially areal coverage.
- Near the Gulf of Mexico,
the NGM very often underforecasts 0.50" to 1.00" rains,
and GROSSLY underforecasts amounts heavier than that.
- Like the AVN, the NGM has
a "cold bias" with respect to the predictions of 1000
to 500-mb thicknesses over sea-level cyclones, during the winter.
- This model can be deficient
with the deepening rates of cyclones across the Gulf Stream during
- This model tends to overdevelop
cyclones in the lee of model terrain features.
- The NGM predicts too much
stability through too deep of a layer with temperature inversions.
- Of all the operational NWP
models, this model can be the most deficient with "digging
troughs" across the western US (the only exception may be
- The NGM has a bias to underforecast
precipitation amounts onto the west coast of the US in the presence
of a "blocking ridge" across the Gulf of Alaska.
- All models, especially the
NGM, forecast too much precipitation across Texas during "pre-frontal
squall line" situations. That is, once the squall line has
moved east and middle tropospheric winds have veered to more
westerly, the rain event for Texas has usually ended. Often the
NGM will still predict rainfall. This bias most true during and
spring and fall/early winter.
- The overall errors in the
NGM decrease during the warmer months.
- C. The Eta Model
- Of the short-range models,
with respect to forecasted cyclone storm track, the Eta seems
to have the most southern bias, especially from east of the Rocky
Mountains to west of the Appalachian Mountains. East of the Appalachians
("east coast storms"), this model can be too far northwest,
even more so than the AVN/NGM.
- The Eta normally is superior
to the AVN/NGM with predicting precipitation amounts from deep
- This model is superior to
the NGM/AVN in forecasting the southward extent and rate of movement
of Arctic airmasses across the middle of the country, east of
the Rockies (especially seen by utilizing model predicted boundary-layer
- The Eta outperforms the NGM/AVN
for forecasting precipitation to the lee of the Cascades and
Sierra mountain ranges (less "spill over").
- This model can be too conservative
("underdone") with moisture returns from the Gulf of
Mexico, which can result in computations of lifted indices to
be too high (too much stability).
II. Biases of the Medium-Range
Numerical Models (Days 4-10).
- A. The Medium-Range
Forecast Model (MRF)
- Shallow cold air is not handled
well, i.e., the MRF will likely be too slow with southward transports
of shallow cold air masses, especially arctic air to the east
of the Rocky Mountains. This problem generally is true for all
the operational forecast models.
- The model tends to have a
wet bias, i.e., areal extents of precipitation often are overforecast,
especially light amounts.
- Often the magnitudes of easterly
boundary layer upslope flows along the Front-Range of the Rocky
Mountains are overpredicted, i.e., pressure gradients are too
- The model has a slight cold
(heights too low) bias, especially for about the eastern third
of the country, with respect to the prediction of mid/upper tropospheric
geopotential heights and resultant thickness calculations. Often
the MRF will dig troughs in the height field too far south across
the Great Lakes and Northeast regions, most noticeably after
about Day 3. During the cold season (mainly October-April), the
MRF will depress the storm track too far south across the Plains
states as a response to this cold bias. Forecasters across especially
the Northern Plains/Upper Mississippi Valley areas need to be
aware of this characteristic and be careful predicting, for example,
snowstorms to "go to their south".
- Continuing on this "cold
bias" problem, the ramifications to the placement of the
MRF model storm track may also be regime dependent. However,
this is a personal observation and requires considerable objective
quantification. For instance, as was discussed during the AVN
section, for the Rockies and Plains States, there may indeed
be a "northern and western bias" in the presence of
a Reverse Pacific/North American Teleconnection (RNA or negative
phase PNA -- western North American trough/southeastern USA ridge,
mid/upper tropospheric geopotential height field -- other regimes
can also project this circulation structure with respect to the
USA -- see reference cited below). The "southern bias",
especially for areas east of the Appalachian Mountains, may be
more likely during a positive PNA (western North American Ridge/eastern
North American trough).
- Somewhat related to this
model's cold bias, the MRF often looses too much "energy"
too quickly for mid/upper tropospheric cyclones translating northeastward
from the southwest deserts. There is often too much "weakening"
in confluent flow.
- The model does have a bias
to meridionally couple separate branches of the westerlies (i.e.,
too much "phasing" occurs between northern and southern
stream weather systems).
- The MRF still has a problem
maintaining correct amplitude to low wave number regimes. That
is, after about forecast day 3, this model has a tendency to
erroneously de-amplify regimes where the planetary-scale is dominate.
An example would be the MRF model "crashing" a western
North American mid/upper tropospheric ridge in the height field
prematurely. In the latter case, the MRF may not be maintaining
enough amplitude to the positive projection of the Pacific/North
American Teleconnection (PNA; see Barnston and Livezey, Mon.
Wea. Rev., June 1987). This problem is important during situations
involving Arctic outbreaks. Also, perhaps related to the MRF's
problematical prediction of the planetary-scale waves, the MRF
can be deficient with "digging troughs" across the
western US during "split flow" situations.
- Still addressing the planetary-scale
waves, especially poleward of about 50 deg north, the MRF has
a tendency to be too retrogressive, most noticeable beyond day
5. For example, this model "likes" to retrograde mid/upper
tropospheric western North American ridges to the point that
the response of the flow regime across the US is a "flip-flop",
from a western ridge/eastern trough to western trough/eastern
- B. The UKMET (United
Kingdom Meteorological Office, Bracknell U.K.) Medium-Range Forecast
- Like the MRF, the UKMET model
has problems with shallow cold air.
- The model tends to be "too
zonal", especially after about Day 3.
- The UKMET tends to progress
shorter wavelength features too quickly.
- Westerlies are often too
- The model likes to lower
sea-level pressures too much and too far south. A result of this
problem can be that synoptic-scale fronts are implied to be too
- This model rarely develops
mid/upper tropospheric closed cyclones and anticyclones. This
is likely a result of its "zonal" bias.
- C. The ECMWF (European
Center for Medium-Range Weather Forecasts, Shinfield Park, Reading
U.K.) Medium-Range Forecast Model.
- Overall, the ECMWF does well
in predicting mid/upper tropospheric heights during the colder
part of the year (such as October through April). Also, the ECMWF
model tends to perform quite well in predicting amplitudes of
planetary-scale regimes such as the Pacific/North American teleconnection
(PNA). This model can also perform outstandingly during low to
high planetary-scale wave number transition events, and northern
hemispheric-scale regime transitions (Berry et al. 1996, CR TM
- This model does the best
of the medium-range forecast models during "shallow cold
- There is a bias in the model
to "overdevelop" mid/upper tropospheric cyclones across
the southwestern U.S. Situations arise where this model will
be too slow to predict the movement of cyclones from the southwest
- This model has a slight tendency
to forecast mid/upper tropospheric heights and the resultant
thickness calculations too high (i.e., a "warm" bias).
- There are some situations,
especially during the warmer portion of the annual cycle, when
this model has too many closed gyres. This bias may be related
to its T216 wave number resolution.
information on model biases is
available from the Hydrometeorological Prediction Center of NCEP.